I was there this year during the marathon (January 6th). The resorts & theme parks were crowded that weekend. Next year the marathon is being held later -- January 12th -- so the crowds shouldn't be as bad. Here is the link to Disney's Marathon.
It seemed quite crowded this past year, however there were still remnants of New Years crowds there.
The Marathon attracts 16,000 runners. There are also about 2000 volunteers. Figuring that each runner has at least one family member/friend with them (when you average it out) that's an extra 32,000 people. My bet is that over half stay on-site.
It definitely brings up the numbers.
We're already booked for the 2003 Marathon. Staying at the BCV
The numbers go up slightly during marathon weekend, but the crowds are still extremely reasonable. Most people that do the marathon are locals or are just there for the weekend. We have friends that drive out Sat. morning, race Sunday and drive home Sunday night - they live in Atlanta, GA.