SNAFU
The front page was b0rked for a couple of hours. I still had Movable Type installed, and it looks like someone managed to send a trackback (I had deleted mt-comments.cgi to prevent spam, since I don’t even use MT any more), which caused the front page to be overwritten by MT.
I took this as a sign that it was time to upgrade to WordPress 1.5. So I did. All is well again.
Later: Surprise surprise, it was trackback spam.