Re: Transmission failing?
About 6 months ago mine did the same thing. Only happenned 3 or 4 times, never did it again.
Keep an eye on your fluid level and if you're due for service (transmission flush), take it to a transmission shop and pick their brains a little while they flush it.
For what it's worth, I just had mine rebuilt (front pump seal went bad). I figured with 150k on the current tranny, I'd better get the whole thing redone. I can't say if that reverse thing was a sign of things to come, but it wouldn't hurt to talk to a shop. Good luck.