I brought my 535i in this morning to BMW San Diego, and my service adviser fixed the loose gearshift knob as I waited. Turns out that either during manufacture or sometime after I had taken delivery the shift knob had simply popped out of the "socket" which the post coming out from the transmission is pressure fitted to. He had a new part to show me what the socket looked like in the shift knob, which you can see when you turn the knob upside down. He applied some manual force to the knob and seated it firmly onto the transmission post, where it snapped into the socket. So, problem solved, and the knob is now on solidly and doesn't rotate at all. My brother will be pleased.