Yes, Thanksgiving 2003 was very crowded. We've been three times Thanksgiving week, and this year was the worst. However, with planning we managed to get everything we wanted done. We used EE, FastPass, and park hopping to make the best of the crowds. We used EE Tuesday morning at MGM- very crowded- and park hopped to Epcot, definitely the right decision! Wed at MGM was not nearly as bad. Thursday at MK is a zoo, but my mom likes to be there for turkey day, so we go. We get there when it opens- we had an early ps this year and they opened the park an hour before schedule- did Fantasyland, parades, etc. Friday was not bad at AK-EE or MGM. It is crowded, but it can be managed! You have to be at the parks at opening to get the main things done and a FastPass or two for other things.
Christmas day through New Years I've read is the most crowded time of the year. I know New Years eve set a record at mk this year and the parks in general were reported to be the busiest yet Christmas week this year. Maybe someone has been during this time that can help more.
I'm a teacher, so I understand about having to go when school is out. We will go Thanksgiving week again this year. We know there will be crowds, but we'll plan and try to accomplish everything we want.
Good luck deciding!