My rear rotors warped at around 25000 miles. It was noticeable as a vibration felt primarily in the steering wheel when braking from about 50mph or faster, especially down hills. Toward the time my appointment with dealership was coming up, the entire car would sometimes feel like it was shuddering. Replaced for free under warranty.
I found it rather odd the rears would warp before the fronts.
2011 E90 / Premium Package / Stage 2 PPK / Performance Suspension