We were there January 1-7, 2005. I thought the crowds were still a bit heavy in the beginning of the week - due to the people who stayed after New Year's. We went to the first night of EMH at MGM and it was PACKED!

Waited 40 minutes for Great Movie Ride (why? Not sure since later in the week during the day it was basically a walk on!)
But, as the week wore on, the crowds thinned out. One morning, we were able to ride Splash Mountain 3 times in a row (5 minute waits), while a friend and her son did all of Fantasyland. We were even able to find her in Fantasyland without a problem. (Usually too packed to move).
Anyway, we enjoyed it so much, we are going this year from January 6-14, 2006. Yes - we leave in 2 days!!!

I am told that Marathon weekend adds to the crowds some. There is also a builder's convention that descends on Orlando every January - but mainly they stay off site.
Hope that helps. My daughter now thinks that "First there's Halloween, then there's Thanksgiving, then Christmas, then Disney!" I like it!