There are many prowlers with this problem, mine is one of them. I have a 08 650xt and they have done this from the start. It is the rear ones that chatter, mine have gotten better with time but have not stopped. Other people have taken them and had them replaced along with the disk and still have problems, but like I said they are getting better and I have over 500 miles on them.
Just purchased a new 09 700 xtx and the rear brake chatters. Didn't know the brakes would do this since I only poked around the dealership on a test drive. I'm really bothered by this. In reading the other posts this seems like something I'm going to have to learn to live with. Dealership wasn't any help. Just said they had put a call in to Cat and would let me know if this was something that needed fixing. Of course there was no documentation of the issue on my service order.