Looking to do the same thing...sorry I didn't get the factory option, and looking to be able to have basic control through the radio, using the radio display. I contacted Dan at United BMW and their discounted 1addicts price on the adapter is about $240. Check it out by clicking on the 1Addicts link on their page.
http://www.trademotion.com/partlocat...?siteid=215011
Dan said that he estimated about 1.5 hours of labor from the dealer as well for installation and programming. I'm going to talk with my service manager and see if he will give me a better deal...maybe they can change the oil while they're doing it!
I've been using an iPod Nano plugged into the Aux jack and it sounds and works OK, but it's a pain to have to fumble for the iPod and use the small screen when driving.