Christmas week itself will crowd the parks, the week beginning Christmas Day and going through New Year's Day. That week is insanely crowded. Stay away.
Other than that the only thing that you may be thinking of is the Pop Warner Football and Cheerleading Championships. That is usually the first full week in December, although for 2008 it's the second week due to the way the first week comes right afer Thanksgiving.
Pop Warner shouldn't really affect the parks that much.
Some companies still have conferences and conventions that use the WDW resorts into about the first half of December but that affects the resorts more than the parks.
Good luck.
:xmas::xmas: