Besides lack of Reverse, do you also have ...
lack of engine breaking in 1st gear?
If so, guess you have D brake ****ed up in the trany and will have to Overhaul it. If you are in USA, there is a guy highly recommended, Kirt Koeller.
Sorry, no electrical gremlins cause this, specially if you do not have the Transprogram warning.
The mechanic that fixed mine said that due to a fault in the valve body, it was applied the reverse and first gear simultaneously damaging the D brake. Also told me that If my wife would have switched the trany to manual and 2nd gear when she noticed the car running braked, we could have avoided mayor damage in the trany.
Javier
PS: Already went trough this, see this thread:
http://www.bimmer.info/forum/showthread.php?t=5487
Engine breaking in 1st means that if you select the ...
Trany on 1st gear, after running (slow speed please) in "D", the car will slow down as a result from the engine and will not release and run free when the transmission shift down to 1st gear. I bet you, you will notice the difference once it shift down from 2nd gear. Also once released, when you push some gas, you will feel a delay in engaging the trany.
Best way to test is down the hill.
Javier