I got married December 18, 2008 and the weather was perfect (high 70's to low 80's). We had a firework party in the evening and had arranged for blankets to be available in case it was cold, but I don't think anyone used them. I was in the strapless dress and was not cold at all in the evening. We were at Disney for a week before the wedding and I don't think it was ever cold during the day, though most of the nights were a bit colder than our wedding night.
However, I used to live in Florida and sometimes in December it can get VERY cold (as in winter coats are needed). You just have to prepare for everything and hope the weather goes your way. The one good thing is that there isn't much rain in December!