Hey... it seems like this is still an ongoing issue. I just got my '11 328xi and it had the wind whistle. Is it doing it for both mirrors for you guys, or just the front left mirror?
Anyway, I brought it to the dealer, they test drove it and "removed mirror cover and sealed air gap to mirror with butyl cord." There was some gap in the seal when they took it apart. The noise is completely gone for now, so hopefully thats a permanent and easy fix!