Stephen,

I had a similar occurence on my M3 with only 3 months/2500 miles on the new Michelins (left rear) at the time. The BMW dealer in my case also said I had to deal with Michelin directly.

Over the phone, the Michelin folks told me that this kind of separation happened in situations where there was extremely low pressure in the tire - essentially a run-flat scenario. While these tires don't actually "run flat", the inference was that the pressure was low enough to cause the stress along the sidewalls to rip the tire apart. At the time, I did NOT notice any difference in handling/noise/etc.

They told me to check the tread of the tire for punctures (I had kept the separated tread face, which was now a conveniently folded oversized black rubber-band in my trunk. Like an idiot, I ran into the traffic lane to retrieve it - expecting that I would need it as evidence with Michelin). Sure enough, there was a small nail through the tread. How long between the time I picked up that nail and the tire wall separated we'll never know - what I do know is that Michelin told me I was S.O.L. since punctures like that were not covered by the warranty.

Other than this incident - I am still a big fan of the Michelins for daily street use - they hold up great for me in all weather conditions, and bite like a son-of-a-bitch. My business partner has the same tires on his '97 911 cabriolet, and swears by 'em as well. The big caveat of course is - Watch The Tire Pressure Closely - lest your sidewalls blow apart!

TK