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If a 10 mile by 5 mile chunk of Antarctica can break off without causing a mega tsunami then I'm guessing the eastern shore of the U.S. is safe.

I would say that the notion of OBL doing something so stupid is off the mark, but as far as the notion of a mega-tsunami from natural causes, I wouldn't be so sure. According to a BBC documentary (which I'm sure is the source of the Discovery piece), if a chunk of LaPalma or some other large island (like, say, Montserrat) fell off into the ocean, large scale destruction would result.

Mega-Tsunami: Wave of Destruction
BBC2 9:30pm Thursday 12th October 2000

Scattered across the world’s oceans are a handful of rare geological time-bombs. Once unleashed they create an extraordinary phenomenon, a gigantic tidal wave, far bigger than any normal tsunami, able to cross oceans and ravage countries on the other side of the world. Only recently have scientists realised the next episode is likely to begin at the Canary Islands, off North Africa, where a wall of water will one day be created which will race across the entire Atlantic ocean at the speed of a jet airliner to devastate the east coast of the United States. America will have been struck by a mega-tsunami.

Back in 1953 two geologists travelled to a remote bay in Alaska looking for oil. They gradually realised that in the past the bay had been struck by huge waves, and wondered what could have possibly caused them. Five years later, they got their answer. In 1958 there was a landslide, in which a towering cliff collapsed into the bay, creating a wave half a kilometre high, higher than any skyscraper on Earth. The true destructive potential of landslide-generated tsunami, which scientists named "Mega-tsunami", suddenly began to be appreciated. If a modest-sized landslide in Alaska could create a wave of this size, what havoc could a really huge landslide cause?

Scientists now realise that the greatest danger comes from large volcanic islands, which are particularly prone to these massive landslides. Geologists began to look for evidence of past landslides on the sea bed, and what they saw astonished them. The sea floor around Hawaii, for instance, was covered with the remains of millions of years’ worth of ancient landslides, colossal in size.

But huge landslides and the mega-tsunami that they cause are extremely rare - the last one happened 4,000 years ago on the island of Réunion. The growing concern is that the ideal conditions for just such a landslide - and consequent mega-tsunami - now exist on the island of La Palma in the Canaries. In 1949 the southern volcano on the island erupted. During the eruption an enormous crack appeared across one side of the volcano, as the western half slipped a few metres towards the Atlantic before stopping in its tracks. Although the volcano presents no danger while it is quiescent, scientists believe the western flank will give way completely during some future eruption on the summit of the volcano. In other words, any time in the next few thousand years a huge section of southern La Palma, weighing 500 thousand million tonnes, will fall into the Atlantic ocean. What will happen when the volcano on La Palma collapses? Scientists predict that it will generate a wave that will be almost inconceivably destructive, far bigger than anything ever witnessed in modern times. It will surge across the entire Atlantic in a matter of hours, engulfing the whole US east coast, sweeping away everything in its path up to 20km inland. Boston would be hit first, followed by New York, then all the way down the coast to Miami and the Caribbean.

From this site:

Appalling as the World Trade Center bombing was, it's absolutely nothing compared to the apocalyptic event that is yet to come. Somewhere over the next stretch of time, New Yorkers will have to deal with a HUGE tidal wave of biblical proportions. Meet the La Palma Megatsunami -- and shiver.

Try to surf this: a wave almost twice as tall as the Empire State Building, and traveling faster than an airplane, at a breathtaking 700 kilometers per hour. Enough to give you the creeps, right? Well, it should. For the question is not if such a massive wave will one day slosh across the oceans -- but when.

And there's more disturbing news. If the Doomsday Wave kicks in, it'll be end of story for the US East Coast. New York City: gone. Boston: washed away. Florida, Miami, the Bahamas -- all history. Oh, several hundreds of years may pass before the Super Wave rolls in. But on the other hand, it could happen next month as well.

It all starts with a faint rumbling of the earth, thousands of miles away. Near the African shore, the volcanic island of La Palma will rumble with seismic activity, announcing yet another eruption of the Cumbre Vieja volcano on the southern half of the island. But this time, the unstable western flank of the volcano doesn't hold. A huge chunk of the island simply breaks off and drops into the Atlantic Ocean. Plunge! No; PLUNGE!

Immediately, up rises a tsunami. And not just a tsunami: the sea is pushed up something like 650 meters, as 500 billion tons of rock and debris hit the ocean. Witnesses to the event will see how a massive wall of water towers up and starts sloshing westwards across the Atlantic, at the astonishing speed of over 700 kilometers per hour. Destination: US East Coast.

Now that will bring about some stress on the other side of the Atlantic. New Yorkers and millions of other inhabitants of the US coastal region will hear about it on the radio: within eight hours, the entire east coast will be splashed away! Hundreds of millions of Americans will get the hell out to escape the wet cataclysm, turning every city and every road in the area into total chaos.

Flush! - By the time the La Palma wave hits the US, it is `only' something like 50-80 meters high -- and twenty kilometers long.

Some people will hope the Megatsunami will just break down by itself. But the La Palma tsunami isn't just the next dull wave. It will pass storms unscathed, and bombs won't do it much harm either. You see, the Megatsunami we're dealing with is an endless wall of water that stretches out across the entire Atlantic! Not exactly the kind of wave that easily breaks down.

Then, after eight hours, the incredible happens. At the US shore, all water suddenly recedes, as it is `sucked up' by the wave to come. You will be able to see the bottom of the ocean for hundreds of meters ahead. But your attention will be caught by something else: this huge wall of water, that's moving in on you a lot faster than you think might be good for your health.

As it closes in on the steep coast, the tsunami will grow and grow some more, until it is perhaps several hundreds of meters high. Seconds later, it impacts. With unimaginable force, billions of gallons of water slam into the coastal areas. Skyscrapers snap, bridges are ripped to pieces and outside the cities, forests and villages are swept away. And obviously, if you're still around, it's only likely you're killed in the whole show. The La Palma wave will push twenty kilometers into the land: since the wave is so long, more and more water will be pushed land inwards, until the entire wave has finally rolled onto the shore.

Next, the water mass withdraws. With unimaginable force, the wave is sucked back into the ocean, dragging with it everything and everyone it encounters. So one moment you're high and dry; only minutes later, you find yourself floundering in the Atlantic, well away from the shore. In effect, the tsunami will wash the coastal area clean. And although it won't really bring about the end of life on our planet, millions will die -- not to mention the total disruption of the world when the urban heart of America is swept away.

And by the way, the Americans aren't the only ones affected. More southwards, the Middle and Southern Americas will be struck. Calculations show Europe will suffer as well. The southern part of England will be inundated. Parts of the low lying Netherlands will be flooded. Some parts of the French and Spanish coasts will be severely hit. In Europe too, many thousands will die.

Like almost every apocalyptic scenario described here on Exit Mundi, the one with the tidal wave is nothing really special in Earth's history. You see, it's quite normal for a volcanic island to collapse every once and a while. That's why on several occasions in our prehistoric past, giant killer waves rolled over the seas, bringing about doom and destruction everywhere they hit land (see table).

Ohau island, Hawaii	Many times in history, big chunks of Hawaii plummeted into the sea. The biggest event occurred 2 million years ago, when a piece of rock ten times as big as the Mount Everest broke off the island of Ohau. Result: a HUGE tsunami, hundreds of meters high and tens of kilometers long.
Cape Verde island	80,000 years ago, the west coast of Africa was rinsed clean, as another massive tsunami hit it at nearly full force. One hour earlier, a massive chunk of the Cape Verde island had plunged into the sea.
Réunion island	4,000 years ago, the island of Réunion in the Indian Ocean snapped in half. Another megatsunami popped up. Several hours later, it slammed into the coasts of Australia.
Water fury: Some (in)famous megatsunami events. On many occasions, huge tsunamis swept over the continents -- these are only a few we know of.

And La Palma? Only recently scientists realised that the ideal conditions for the next landslide exist on the island. Actually, in 1949, an eruption of the Cumbre Vieja almost triggered the La Palma disaster. The volcano erupted, the weaker half of the island dropped down four meters -- and halted. It was only in the nineties geologists realised what a close call it really was.

No-one knows if the island is able to withstand the next volcanic eruption. Gladly, we'll know several weeks in advance the Cumbre Vieja is about to go berserk again. But then again, will our governments take the warnings of the geologists serious enough if it gets that far? Will they be willing to evacuate the entire US East Coast and large parts of Europe, following the advice of only a handful of scientists? Somehow, Exit Mundi doubts it. Perhaps the killer wave will take us by surprise after all.