I just recieved a MINIUSA parts bulletin detailing the release of new magnetic stone guards for the MINI. For those who are familiar with the BMW Z3 stone guards these are very similar. Here's the info from the actual MINIUSA bulletin (You can see the entire bulletin here):

Aftersales is pleased to announce the release of the Magnetic Stone Guards for the MINI. The Magnetic Stone Guards will protect the area in the front of the rear wheel against stone chipping, enhance the sporty design of the car and blen nicely with the black fender flares on the MINI. The stone guards can easily be installed by a customer. The kit includes two stone guards and detailed installation instructions. The MINI magnetic stone guards are only available in black

The part number for the stone guards is 82 11 0 304 813.