Yes I have done WDW over Thanksgiving. One year I chose to return home Saturday night nonstop rather than take the more expensive Sunday night nonstop or the not more expensive Sunday connecting flights, fares as of when I booked. Another year I returned home the Wednesday after. The sunday eve. plane fare for the best flights shot way up well before a third of the seats on the seat map were claimed.
Nov. 2003 I plan to be at WDW again over Thanksgiving, having snagged a good fare including Sunday evening back. So I expect to see for the first time how crowded MCO is on Sunday after Thanksgiving.
On many of the choice flights to and from WDW around Thanksgiving and other peak dates, the cheapest seats are gone 331 days in advance. Within two weeks thereafter the price can reach the maximum, or Y booking class. There may be periodic reductions over the months but generally not by many dollars.
There is constant traffic to the airlines' web sites by those who missed the initial offering of cheap seats and hoping to hit not so much an across the board fare sale but rather another allotment of cheap seats. The if and when of such an offering is normally completely unpredictable. In 2003 I observed that Delta (... Express) had a second offering of cheap seats for Thanksgiving dates in mid January when Boeing 757's were substituted for 737's on the BOS-MCO run, a third offering on the JFK-MCO and BOS-TPA runs when Song began booking on 4/1, and probably a third offering on the BOS-MCO run when Song began booking between those cities ca. 4/14.
The airlines know that they can fill the planes on the choice dates for "full fare*" eventually and also know that the entire plane will sell out 300 days in advance if all the seats were cheap. So they only offer a few cheap seats at first. I do know for a fact that Delta and US Air did have some cheap seats (about $110. one way MCO-BOS) for Sunday eve. after Thanksgiving and I conclude they had some cheap seats for every single flight 331 days in advance.
Travel tips:
http://members.aol.com/ajaynejr/disney.htm
* Delta Express has had a non-refundable Y fare typically just under USD $300. each way (BOS-MCO) as well as a refundable Y fare of about $600. each way.