I had the very unpleasant experience of a DVD drive seemingly dead with a DVD inside: I inserted a DVD in the drive, but it didn't mount and I couldn't eject it. I restarted the Mac while pressing the mouse button down (both the track pad and then a USB mouse). Nothing. I restarted again and pressed Command-O-F followed by eject cd. Nothing. More worrying: System Information wouldn't see the DVD drive at all. I restarted several times with no more luck. It looked like it was the end of the DVD drive, right?
Wrong. I just had to put the Mac to sleep by closing the lid of the laptop. When I woke it up, the drive woke up as well, the DVD appeared, and I was able to eject it. I thought this hint, albeit totally straightforward, would be useful because I was very close to running to the nearest Mac support store. I would never have believed that putting the Mac to sleep could be the solution! I'd be interested in any rational explanation of this behavior.
[robg adds: Note that this is a different issue than this one, which dealt with discs stuck in optical drives. In this case, the media's not stuck, it's as if the drive itself has vanished from existence. Similar to the last hint, though, I've not had this happen to me, and would be curious to know if others have had their drive seemingly vanish.]
Mac OS X Hints