We spent 8 nights in WDW at the end of Jan. 2006.
The mornings/evenings were cold. I'd packed hats & mittens for us & we did use them in the early mornings.
I'd packed turtlenecks to wear under our WDW sweatshirts & DID NOT use those. During the day (a few days, we just kept our jeans on) we wore shorts & even went to Blizzard Beach 3 times...but be warned, the water took some getting used to.
You need to be prepared for warm & cold....as you just won't know until you get there.
Have fun!!