The MyBB installation wizard is telling me uploads/ and uploads/avatars/ aren't writable (ie. chmodded to 666), but they are! What's wrong with this?!
Sorry about that. The documentation needs to be changed. They should be CHMOD'ed to 777.
[Wiki: CHMOD] (Broken link, head over to docs.mybb.com instead)
Thank so much for such a speedy and helpful reply, Tikitiki.