A very strange, but limited bug, that applied ONLY if you had Guest turned on, had been logged onto to the Guest account and did not log off Guest before doing the update to Snow Leopard, and then immediately logged onto the Guest account before logging on to any other user after the update. . . something no one I can think of would do. . . but someone did it.
The Guest Account is designed to delete all data placed in the Guest User space during a session when logged off. That way a new guest does not see what the previous guest was up to. This bug accidentally got confused about which space it was in.
The accidentally "deleted" data, it actually was made inaccessible, was easily recoverable. Of course, backups are always important.