My dh and I were at WDW this past January 2-12. Our previous trip there was also in January, but a week later (7-16, I think).
Here's my take on things:
On January 2 there were about a zillion people in MK. Since our previous trip had been slightly later in the month, we were quite surprised. In fact, until the weekend following New Year's was over, there were far more people in all the parks than we had experienced on our previous trip.
We had purposely scheduled our trip to be there the 1st weekend in January, though, so that we could see the Osborne Family Lights (which aren't up this year, btw), the Xmas Country Bears show (it was adorable!), and ... of course! ... Carousel of Progress -- all of which we got to see. And, as a bonus, it was really wonderful to see all the Xmas decorations in the parks and the resorts.
After the weekend was over, though, the crowds were nonexistent and we were back to our happy walk-on-to-every-attraction theory of touring WDW. Even the Marathon weekend didn't really affect park attendance that much.
So, I guess what I'm saying is: Based only on my January 2003 experience, it's still crowded after Christmas and doesn't really let up until after the 1st weekend in January. If I'd known this in advance, I'd've made some PS's for dinner for the first 3 nights or so (by Sunday, it was empty), because on the 2nd we ended up stumbling around the MK looking for a counter-service place (I was dizzy with hunger! lol) because, after our previous visit, we'd just assumed we could stroll into Tony's and get a seat! Wrong!!
HTH --miffy--