Jeff, I don't know if you recall, but the forum discussion actually started here on the WP Tavern forums. I had finished the plugin for a couple of clients at the time (who I eventually moved to bbPress). JJJ really helped me nail down the concept for the plugin.

I like bbPress, but I still don't think it's the solution I want to see. This is just from a personal viewpoint. It's a great plugin though.

Like JJJ said, ideally forums would be a custom taxonomy rather than a post type. To me, the ideal structure would look like this:

* Forum - taxonomy
* Topic - post type
* Reply - comment type

When you get right down to the meat of it, forums are essentially no different than a category/post/comment structure, just like blogs. The big hurdle with WordPress handling forums like this is term (taxonomy) meta. There's a good bit of tinkering you'd have to do to make comments become "replies". Then, there's some query-related stuff, but WP has gotten a lot better in that area in the last few years.

As far as this theme is concerned, I don't like the idea. Forcing users to have separate installs is going to automatically lose you a ton of potential users. You already know how I feel about themes creating post types, so I won't get into that. If they had created this as a standalone plugin, I would've definitely installed it and even considered building themes for it.