Winter permits are really easy to get, there are cancellation lotteries for those times all the time. Actually, one just ended. If you sign up on the GC permit website for email notification, they'll contact you when there are cancellation lotteries.
The recent dates available were: Nov 2008: 30, Dec 2008: 3, 9, 11, 13, 20, 24, 25, 30.
Based on the statistics, I don't think they usually fill all the winter ones.
I am a river, babe - I've got plenty of time, I don't know where I'm going, I'm just following the lines..... - "We are water" by Shaye