Well, at least it’s not just me. And the upgrade to the final release of 3.0 didn’t fix the problem, either.
But I went back and looked and it turns out I’d overlooked that checkbox. I enabled the forum, and now it’s working. Whew! I have a prospective client who wants a social network converted from a proprietary CMS to BuddyPress, and it would’ve been tough to do that without a working forum.