Is this a conference or a trade show that you're talking about? If it is, I'm pretty sure attendance at those types of things dropped off pretty substantially after 9/11, so it wouldn't be totally irrelevant to make the comparison.
That's not to say that you don't hear a lot of spurious stuff attributed to 9/11 or the pre/post-9/11 "world." In my business (magazine publishing), I heard ad sales people using 9/11 as an excuse for not meeting their sales goals for more than 2 years afterwards. At first it sounded reasonable, then it sounded lame, and finally it started sounding a little blasphemous.