We went in Jan 7-15, 2010. Crowds were light at the beginning, especially considering it was Marathon weekend. They picked up, especially on the 15th, as it's President's Day. I guess some schools have it off? Every day, other than that last Friday, we walked onto every ride, and just walked through the queues. I'd trade the weather for the crowds any day. I can deal with the weather much more easily than I can deal with the RUDE!
As for the weather, It was a freakishly cold year in Orlando. The headline of the paper on our first morning there was that it was actually warmer in Alaska than it was at Disney World. Luckily, we had brought our coats (We're from the midwest - DH, DD7, and I all have the 3-in-1 coats by columbia), hats, and gloves, and started out wearing it all the first day or two, and started shedding a little more each day. By the end of the week, we were in jeans and t-shirts. It might have been in the low 30's, but to us, that was still much better than the single digit temps and 10" of snow we had back home.
For our upcoming trip, we're planning on Jan 27-Feb 3, 2012. We chose this time of year again for the low crowd levels, the lower pricing (staying at WL with
DDP for $3300 for 3 of us, including everything but airfare, tips, and souveniers).
For good, detailed crowd projections, check out easywdw.com. They even give you park suggestions for each day, etc.
Good luck!