You will be fine until December 21st. Since the 22nd falls on a Saturday, I bet the crowds will be in full force from 12/22-1/1/08.
We have been the past 3 years around 12/14-12/22 and it has been fine each year. A wait of 30-40 min. for the most popular rides like Soarin or RNRC is not long, come in July!. For those who only go in Sept, or October it feels like a long wait but thats nothing.
I wouldnt pass up the chance to visit WDW during Christmas as it is so beautiful. Dont miss the chance to visit the resorts to see the decorations, especially GF,WL,AKL,BC,YC,BW. Its worth a 1/2 a day.
Also cant misses, are Osbourne Lights, Candlelight Processional and Holiday Wishes.