What are you guys having them replaced for?
The left stick ball thing(TM) on mine popped "up and down" like it wasn't secured properly. Happened after a while and I just kind of lived with it. But I eventually just stopped really using it and didn't bother calling since it was well past the 90 days.
After talking to some of the MS people at E3 they said to just contact support and they'd take care of it, and sure enough they did.