Comments are back, so Chris P., you can now comment on my GarageBand-created track.
The solution? Turns out I had a blank line in my Movable Type banned IP list for retrophisch.com. This was blocking all IPs from posting. Deleted the blank, rebuilt the site just for good measure, and happy comment spamming days are here again.
Thanks to the many posters in the Movable Type Support Forums for commenting on their own comment problems. One of those forum members pointed me to the solution.