I had the rear brakes replaced under warranty on my 09' with only 20K on her.
Didn't even notice a dash warning to the fact - I was in for other warranty service and they replaced them at the same time.
I though it was funny that rear brakes would wear before front - as usually the fronts do more of the 'work'. Apparently, with the way BMW uses the rear brakes for DSC and such - they tend to wear faster...
2009 328ix E92//SGM//ACS Roof Spoiler//MTech Aero Lip+Trunk Spoiler//M3 Side Skirts//CF Roundels+Grilles//M3 Novillo Interior//Perf Shift Lever+Boot//MPerf DCT Wheel//M3 GT3 eBrake Handle+Boot//ACS Pedals//M3 Mirrors//LCI Tails w/LUX Rev//LUX 160 AE+MTec 6000K D1S+HPB HID 6000K Fogs//HPB LED Interior//RPI Scoops//CF Euro Airbox+Underhood Hsgs//K&N//BMW PE//MTech Rear Bumper-iS Sport Diffuser//CF Strut Brace//Vishnu OFT//BMW 193M's+Kumho 4X