Here we are - a bunch of Canadians visiting Universal Orlando in December 2010.
Yes, it was cold. We were very surprised to find ourselves still in the winter jackets and toques we'd worn to the airport!
However, it was also the BEST vacation you can possibly imagine, and if I were to be visiting again and heard that a cold snap was expected, I'd throw a party on the spot.
Why? Because this was right after Hogsmeade opened (remember the lines all the way out the highway?) and despite that Universal Studios was
deserted. Almost no lines for anything, and hardly anyone anywhere, even in the Harry Potter section of the park. The place was like a ghost town in the mornings. Ever fantasize about having the parks all to yourself? Well, this is how it happens.
As far as practical advice re dressing warm... I highly recommend warm hats and light gloves. You don't need a heavy coat, and in fact layers (t-shirt, shirt, sweater, light windbreaker) will give you a lot more flexibility to adjust if you start feeling overheated. You don't want to start sweating when it's cold out - that'll chill you faster than anything. And if you decide to wear long johns, count that as an extra layer - ie, if you would normally wear four layers up top, you can cut it back to three when you've got long johns on.
Scarves are completely unnecessary (we were just showing out love for Hogwarts!).
Honestly, you sound well prepared and I think you'll have a marvelous time!