Sorry to dig up an ancient thread, but I think this is pretty funny.
Everybody I've ever met has called BMW's Beamers or Beemers, or however you want to spell it.
Beemer is certainly the accepted pronunciation in most of the US, but after looking around the web I see that there are a ton of elitist BMW-enthusiast freaks (mostly on the east coast) who demand that their cars be called Bimmers.
It's actually sad.
I found this site, where a guy from a Boston "Bimmer" club contacted BMW. The spokesperson actually says that "Beemer" has been around longer, and while "Beemer" and "Bimmer" are both accepted, there is no explanation why many people call the cars Bimmers. The "Bimmer" club still tries to turn this around and say that those who call the cars Beemers are idiots.