Helen,
I think that December is a fantastic time to visit! Right up to the week of Christmas the crowds are fairly light. They get to their very busiest for those last two weeks of the year, but beforehand it is just a dream. And of course you get the added perk of the holiday decor everywhere.
As to the weather, it can be hit or miss. I was there in 2008, and it was fantastic weather, certainly warm enough to swim. 2009 was not so great, but mostly from rain, not necessarily due to cold temperatures.
The National Weather Service reports that Orlando temperatures in December are as follows;
- Average Temperature: 63.0 degrees
- Average High: 73.3
- Average Low: 52.6
What you really need to look at is the average high. And the pools are heated at Walt Disney World, so you stand a decent chance of having swimming-worthy weather. So now you just have to leave it to fate to see how the weather will pass in 2010.
I am glad to answer any follow-up questions you may have!
Have a magical vacation!