Given Atta wasn't constantly being watched...
It isn't that difficult to appear to be in two places at once.
I haven't seen the evidence that would suggest he was for certain in the United States on the date(s) in question.
Its very possible using fake passports and other methods that someone could fly to the Prague have a meeting and return all within a 24 hour period, and do so completely undetected.
Again, this is especially true if that individual is not under constant surveillance...(And assuming that constant surveillance is even full proof to begin with)
Of course was it Atta or was it someone else? Who knows...
Scott Evil