Doesn't anyone do a search anymore?
There are a bunch of reasons why the car won't go in reverse. You can pretty much rule out electrics as causing the problem, as Javier said -- the dash is acknowledging the gear selection, and the tranny computer (AEGS) would dump the well known "TRANS PROGRAM" on the check control.
It's a physical problem. The known A clutchpack failure (from idiots revving the engine in neutral or park after running the car in drive) is a possibility, but unlikely since the car has forward gears.
Sounds like my transmission problem, and hey - a year ago, I was going through exactly what you're about to go through! Lucky for you, my less than 1 year old ZF reman, with the remainder of a two year, unlimited mileage warranty, serviceable at any BMW dealership covering parts AND labor, will be available for sale within a week.
http://www.bimmer.info/forum/showthread.php?t=8771
What's the condition of the fluid? How many miles on the car and what kind of service history is known/in writing on it?
best, whit