My husband and I were there 12/22/07 - 01/01/08 and the weather is hard to predict like the previous poster said. There were days where it was 85! We wore jeans/sweaters (or I did) in the evenings, but during the days it was SO warm. One day we actually left the park because we were minding the heat a lot. The temp's changed really quickly though because the day we left it was low 70's if that, and then the next day it was only in hte 50's.
I would pack a sweater, a light jacket, some shorts, a couple pairs of jeans and definately your bathing suit! We're going again this December, and I'm going to be watching the temp's leading up to our trip to decide how to pack!