We have visited in December for the past three years. In December 03 and 04, we went the second week and last year we went the first week. This is overall a slower time for the parks, but we noticed a significant difference in the crowds last year. I don't know if it was due to an increased marketing/discount campaign by Disney, or if that it the normal crowd level for the first week in December. It certainly wasn't a summer type of crowd level, but we noticed it in our increased wait times.
It was still a wonderful trip last year, but as my DD8 has autism and waiting is a difficulty for her, we are going the 2nd week this year.