It all depends on when Chistmas vacations end for schools around the country. This year, many schools didn't start back from Christmas vacation until 1/8, so the parks were still very busy until then (we were there from 1/4 to 1/10, and there was a marked difference in crowd levels after the 7th). I believe this coming holiday, since Christmas and New Years day are on Tuesdays, lots of schools won't start back until AFTER the first weekend in January (Jan 7th).
The Disney Marathon is also usually the first weekend in January. The parks aren't affected too much, but the resorts fill up with Marathoners and their families. The Marathon will be held the second weekend of January in 2008- 1/10 to 1/13.