This is the thread on it.
http://www.e90post.com/forums/showthread.php?t=687463
You have to be sure to get the radio with the most bus for L7(and if you have one and have stuff attached to it like PDC I think) . The coding seems to be the tricky part, and you have to do that yourself or find somebody that knows how to do it. BMW will not do this retrofit for you as far as I know.