Well, I really have my doubts that it's just the fluid but I'm willing to see if that's really the case. I just bought a 2000 323i that has the GM trans a couple of months ago and it started very, very randomly not engaging into reverse. I bought it from a BMW dealership and it came with an extended warranty so I took it up today and had them check it out. Of course they couldn't get it to fail and it isn't showing any fault codes so they didn't want to mess with it. They checked the fluid, etc and sent me on my way for a $118 diagnostic fee, whatever.
But let's see if the fluid fixes it. I know someone who rebuilds transmissions and is an atra member and is actually next door to where I work. I'll have him drain the fluid, flush it, and put in new and we'll just see what it does. I hope that does work, it would be great... but I really have my doubts.