That why this is more likely a diversion to make DHS think airline hijackings are still being planned. Thereby redirecting resources away from the search for dirty bombs and border enforcement issues. They have to know that their chances of successfully pulling off another suicide hijacking are tiny.
I might be wrong, but I think if they find an airport they can penetrate, they will send one person on each plane. That assumes they find a way to get a complete simulated bomb through with one person. The one person gets on a plane and detonates the bomb. If I were them I'd do it during climbout...though there's little risk of discovery at any point in the flight, why risk it? Put the bomb in the overhead on a short timer, or put it under the seat, pull it out partway into climbout and push the trigger.
Consider what they could do if they took those 40 people pepsionice was talking about and put them each on a different flight...all planning to detonate at a certain time. Some detonate during climbout, others during cruise, others during descent. All of a sudden, 40 airliners disappear.
Now, what if they have to bring it aboard in pieces, as the Heathrow bombers planned? Well, then you do have teams...but the object would be to not look like they were traveling together, and only one member of the team would have to know how to make the bomb. One bomb-maker and a handful of females who could bring stuff into the bathroom in their purses. The females could even wear Western dress...if the 9/11 hijackers spent their last night on Earth bozing and getting lapdances, why quibble over whether a martyrette is going to wear the hijab on her way to paradise?
If such a team needs 5 people, pepsionice's hypothetical 40 means 8 dead planes...imagine if they're all cross-continental flights on a heavy traffic day. We could be looking at 2,400 casualties. If they use the one-person-one-bomb 40 plane approach at any but the largest airports, some of the jets would have to be regionals, but we'd still be looking at 5-6,000 dead...even more if most of the detonations happen on climbout or descent and the debris falls on populated areas.
A hijacked plane makes a better weapon, but the hijacker can be stopped by a dogpile, and a hijacked plane will almost certainly be shot down if the hijackers allow the Air Force any time at all to find them. But if the bomb can be gotten aboard at all it probably won't be stopped.