If you have a HTC Diamond, or another phone that BMW does not sell a cradle for, read my post on building your own.
It's not that difficult and you can use any BMW bluetooth cradle that you buy on ebay for example.

http://forums.e60.net/index.php?showtopic=60462

Seems very nice. I went down a slightly different route with my HTC TyTN and TyTN II because I wanted to split the USB to provide audio as well. And at the time the cradle I had was not charging the TyTN because of the limited current. However I never thought of adjusting the Voltage and Current on the board. I learnt something new today. I'll probably do this one day for my phone. So does the Diamond have an external antennae socket behind? I couldn't quite make out how the Diamond was making contact with the aerial plug.

No plug for the antenna, but the original Nokia 63XXi did not have a dedicated antenna plug either. Still the phone is coupled to the BMW antenna (shark fin). The idea is to place an antenna very close to the phone antenna on the back side, maybe 5mm or so (with plastic in between). I did the same trick for the Diamond, except that there's slightly more distance so I'm not sure how good the coupling is.

I attached a picture of the cradle antenna FYI.

Of course a dedicated antenna connector is better than this solution.