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.]

