Here's a Link to an Orlando Sentinel Article about the attendence at Florida Theme Parks. The short version is that the numbers seem to be down from years past, but that they're up from the post 9/11 slump.
For December 2001 and beyond, airlines are gradually adding back flights, but between September 11 and Thanksgiving, choice flights were hard to come by thus cutting down on theme park attendance.
What it was was not enough planes to bring all the people into Orlando and take them back home again making the best use of their vacation time.
Perhaps the theme parks and hotels might want to get together and set up a discount plan for folks who can't get in until late evening and ahve to leave early in a morning. Such as Unlimitd Magic giving a 50% discount off the first and last nights and basing park admission on just the days in between.
I could buy that arguement if the flights were not flying at 60-80% vacancies. People were afraid to fly, and afraid to spend. That is what caused the problems for WDW, Universal, DL, etc.