Not a prediction, just a guess--the date will be 7-7-07.
The terrorists have had a severe dent put in their fundraising after 9-11. Not even Bin Laden has enough money to fill the gap.
They are not ready yet.
They've done a good job of capitalizing on the left and the media's anti-war, anti-Bush spin, and they'll probably want to use that momentum, so I don't see them waiting until a new president has come in. It may be that we see a "softening up" in the PR department first, with some carefully timed articles designed to drive a wedge into the population, and between the U.S. and the rest of the world - even more so than exists today.
I would say, look for a media blitz raining blame on Bush and his policies in advance for whatever happens to us.