I recommend keeping an eye on the daily temp throughout the last half of Nov and beginning of December. One yr we were at WDW and it was in the high 50s, low 60s, but the wind was chilly. There were no gloves or hats anywhere, sold out! I bought a pair of MM gloves that were big and you could't do anything with them on, but they were warm. Now my DD who is 17 months walks around with them on doing her version of "Patty Cake".
We were just there in Dec 2007 and it was very warm. Shorts most of the time, but we wore jeans and sweatshirts in the evening when it cooled. So pack for both. Bring a couple of pairs pants and sweatshirts...or you can always pick up a nice souvenier (sp?) at the parks to wear home!
We are going back this year and can't wait!