Update: Cnet reports that Apple is acknowledging the problem, while calling it "extremely rare."
There's a few threads on Apple's Discussion Boards describing a problem that's affecting some Snow Leopard users. An issue with the Guest account feature in SL appears to be chomping down on user data with extreme prejudice. Cnet/MacFixIt first took notice of the issue back in early September, but a 2nd report & a link from 9to5Mac have raised the threat level quite a bit.
According to affected users, the guest account's logout wipe -- a standard feature intended to prevent guests from leaving files on the machine -- is inadvertently clobbering the home directories of the non-guest accounts, with catastrophic results. One user writes, "I hadn't used my Guest Account since upgrading to snow leopard, and I accidentally clicked it instead of my user account this morning, to find that when I logged into my normal account ALL my files, settings, mail etc had been reset."
Worse of all is that he's not the only one.
MacFixIt reports that it isn't an epidemic, but readers have described the same issue in the comments to their post. For now, we offer this simple advice. First, make sure your backups are current. Secondly, turn off the Guest account feature if you're not using it (simple instructions here); some suspect that the issue is triggered by having it turned on prior to the SL upgrade. If and when the culprit is identified and squashed, we'll let you know.
Thanks to everyone who sent this in.