I think I'm probably the first to have this happen.
My wife was driving the M3 while I was taking my new rims to the tire installer, and called me to report that after taking off from a stoplight, it would not come out of 2nd gear anymore. She has never owned an automatic car in her life, knows what she's doing and so I believe that's exactly how it happened.
Anyway, I managed to take it to the dealer (not far away) using mostly 2nd gear. At times I was able to shift in 3rd/4th, but it was always a struggle to put it back in 2nd, and it never went in 1st. I didn't want to force it.
At the dealer, of course it was in one of the "it works" phases, but he definitely believed me and got me a loaner. This morning they were able to experience the same symptoms, so now they're waiting for instructions from BMW.
My gut feeling was that perhaps some part in the shift linkage had come loose, and when it was displaced the shift lever could not move the linkage sufficiently to cover the 1-2 plane. Then driving the car could bump the parts back into proper alignment and it would shift ok.
I think I'll call them tomorrow to ask them to investigate this possibility rather than try to get a new tranny, which would probably take forever.
Of course, it could also be some obscure internal tranny failure, in which case they'll probably replace it.
The only good news is that they gave me a 135i loaner instead of the usual 328 or X3 - the 135i is a little ball of fun. Props to VOB BMW so far, they are taking care of me.
I'll keep you guys posted as to how this unfolds, may be of interest to other owners of manual transmission cars...