Except it wasn't. The line was gone. The crontab file was empty. Maybe I forgot to save it before I did something else and rebooted the box. So I put it in again. Saved it this time.
And I'm still getting "outdated database" warnings. Huh?
It turns out, crontab was quietly dropping the changes on the floor. Creating a .vimrc in /root with the line "set backupcopy=yes" fixed everything.
Why this is relevant, I don't know.