The UG crowd calender has the third week of December as on of the slowest weeks of the year, but with the Disney Visa Free Dining offer going on December 14 - 20, I'm sure that will bring in some more crowds.
Historically it has been one of the leastest busy times. We were there this same week in 2006 and it was GREAT!! We had a wonderful time, now the next week was a completely different story! But as above poster said the Free Dining offers for that week may really be effecting that this year. I know we have been trying to add on additional night to our ressie for December 13th and I am having no luck whatsoever so that is kind of a scary sign!
It's a great time of year to visit and and one of my favorite times in the parks. With all of the holiday decor and entertainment going on you have a much better opportunity to experience all of the great holiday offerings!