After debugging the bloody thing for two hours at a friend's, I discovered that the source of the problem was the DUMB Debian maintainer, who set `nnmail-spool-file' to "/var/spool/mail" at a 00debian.el file. That setting didn't do what he expected, only he never bothered to CHECK. If he at least used a simple SETQ, it would have been trivial to find it. This way the only clue that anything was wrong was provided by `symbol-plist', after I remembered to check it. Sheesh. What a bunch of Penguinoid losers. :-(