I don't think they'd have any chance of getting away with it on 9/11. Security is going to be really, really high everywhere. I think they're more likely to try for sometime when they think we have let our guard down and are distracted with something else. (Election Day, for example?)
The problem with chosing election day is they aim to affect the elections, so they'll pull something first methinks. I'd think they'd go for the Spain effect. I'd bet they're hoping the Dems will try to impeach Bush. Depending on what they do, if it appears that Bush has not protected us here, it could easily turn the election on a dime.