Usually when there is snow on the ground There will be a high demand for bikes in the spring time. During the fall people tend to start selling and in winter you can find some good deals. I got mine in November of last year for a pretty good price. Yeah it sucks waiting all winter to ride, but the money saved was worth it to me.
I actually think the best time is the fall. That's when people start to worry about how they are going to winterize the thing. Also, like Joberly said, the winter is good because people start wondering why they are making big payments on a toy they can't use.
No, I don't think the difference will be in the thousands of dollars, but perhaps in the few hundreds. I think if a guy was going to have to take it in the shorts to the tune of a thousand dollars, he'd keep it under the eaves of the house.
you can find some damn good deals around January.. Guys start getting antsy to sell their old stuff and buy the new hotness... I've bought two bikes in January in past years, and paid $1500 - $2500 below value for them..
But dont look just in the city papers, go to some of the more rural suburbs and check their papers, better deals than in the cities.