My family and I just returned from a trip that was in mid December (8th-15th) and the crowds were very manageable. At first we were disappointed with the crowd levels throughout the parks, but truthfully the line waits were only 10-20 minutes. The safari in AK was the only line we waited 40 minutes in. We went Dec. 18 - 23 in 2005, and though the crowds did increase as it got closer to Christmas, the parks were still very manageable. We had the same line waits - 10-20 minutes for most. To be honest, I think the secret is out about how nice December is. I think each year the crowds are going to increase, and it is going to get harder to predict the crowd levels and the best weeks to go.
Just plan around what works for you and you'll have a wonderful time.