Your bias is duly noted and noticed.
Here's my most important question: It sounds like 3.1 is far enough off that I can not worry about it for now, but do you know if 3.0 mods would still be installable, or would an existing, working 3.0 mod be forced to recode in order to be installable in 3.1?
I'm not sure why you think it doesn't have that much to do with PHPBB. It's creating and editing posts, and should be managed within PHPBB and fit within the PHPBB mod framework, so i was thinking that this requires extensive knowledge of the PHPBB world.
As to whether or not this is a bad idea in general, I'll have to completely disagree with you for a number of reasons. First (and foremost) our current listserver <-> forum system generates 100s of thousands of pages and millions of resulting pageviews for our site that generate both significant traffic and google ranking. Second, in the world I live in (IT admin), listservs are still more popular than forums, and way more popular than groupware-type sites like facebook that many people are replacing forums with.
Google groups. No thanks. Yeah, that's what I want to do, surrender my site's millions of page views and revenue to Google.
Forums are great and some people love them. Listservs are great and some people love them. A gateway lets each of those groups have what they love and still communicate with each other.