Thanks for the replies. Think I'm coozed.
If this happens to anyone else, there's some good info here about it:
MS file corruption I suspect that since some of my folders were so large/overdue for a cleanup that the compression corrupted them. Tried a couple of tools that claim to sometimes be able to restore corrupted .dbx files, but no luck. I think I dicked around with the files too much first. (Moving 'em, making backups, attempting to open in notepad etc.)
The lesson is clear.
Delete unwanted files, and empty deleted folder routinely, so they don't get too large.
Backup important files as they become important.
And if this happens, shut down background applications, and have OE compress all folders again.