I did a December trip in 2009, and I am going back this December. There is a small group of people that get winter permits every year. There are definitely some advantages to going in the off season.
When I went the average temperature was in the low to mid sixties during the day, and low forties at night. Only three nights out of 19 were below freezing. There was a total of five other groups on the river while we were there. A couple of the groups had people who do a lot of winter trips, and they said that on average the winter I was there was slightly colder than usual. That being said, prepare for the worst weather. People do get trips that fall into a 20 day cold snap.
The biggest advantage was the lack of people. Ranger Dave told us that in the peak season there are 10,000 people a night camping in the canyon. Including our trip, we had to compete with 47 people. There was ample raft parking at all the points of interest, and competition for campsites was non existant. We were always happy to see another group on the water. By comparison this was my trip leader's fifth trip, and he said on every other trip there was animosity between groups over space.
The downside to a winter trip really was not the temperature, but the short days. You have to plan your day pretty well to both make miles, and see the sights.
The only thing you can burn that you didn't haul down the river with you is drift wood. It was plentiful.
Don't give up on your dreams Jon! Remember, most of SI's swimsuit issue is shot in the winter!
Kyle