Funny thing happened when I let my Macbook update itself to 10.6.7 - I think Time Machine was running when that started - and when the machine rebooted, the backup drive was corrupted. The data was still readable, but it wouldn't mount with write capability. The archives (it was only a few weeks worth) didn't matter that much to me, so I just reformatted and started over.

Now, I don't know that it was Time Machine - it's not like I looked. But I can't think of any other reason that the drive would have gotten into a bad state, unless it had a huge problem - and the fact that it's still quiet and reformatted cleanly seems to indicate otherwise....