Most BMW's have a staggered-width setup, where the rear tires are wider than the fronts. If your setup is like this, you cannot really rotate the tires, even side-to-side. The car wears the tires down from the inside-out so as Big D stated above, you do not want to disrupt the tire wear.
If it makes you feel any better, I just purchased a new set of tires and I have almost 24,000 miles and they really should have been replaced one or two thousand miles ago.