Yup, I am getting pretty good mileage out of mine (150/70-18), but I have had these cracks in both my second and third tire.
I am traveling through the Americas and in Brazil right now, so because of my concern I sent a message to Heidenau through firstname.lastname@example.org
and actually got a response. Here is what John B at Heidenau had to say:
We seen these before and though they look bad, haven't posed a threat, often come about from over-inflation. Where they've statistically turned-up a lot, has been Alaska. Riders would arrive at our dealer in Fairbanks after coming up from the lower 48 and have these same cracks. The shops there found the tire inflation at 45lbs or better when the tire is designed for 36lbs. Since they were running loaded and BMW says 42lbs the riders were over-inflating them. The combination of heavy load, long stretches of freeway, and to much pressure would cause cracks.
If I was out in the middle of C.America I'd change it to, but we haven't had a failure due to these cracks.
I didn't over inflate so I took him at his word that they wouldn't cause any problems, and so far they haven't.