(This post is ten years late. Nobody cares any more, including the people who care.)
Old Usenet hands contemplate web forums with aesthetic revulsion. They're a single point of failure, the interface is not flexible, killfiles require Greasemonkey scripts or similar rubbish. Usenet was so much better. (Except even its fans don't bother much any more.)
An NNTP backend on a web forum — or, rather, a web forum interface to NNTP — seems a simple enough idea. So why hasn't it happened?
I think the two key problems are:
On web forums, threads are a first-class entity. On NNTP, a thread is an ad-hoc view of a group of messages, assembled on the fly from what References: headers can be found.
Web forums are synchronous: you will never get a thread with missing posts. Missing posts are normal on NNTP, as its distributed nature means receiving a message at all, let alone receiving it in the right order, can't be guaranteed.
These are technical differences, but they're fixed in place by user expectations.
To what extent would bolting an expectation of strong threading onto NNTP break expectations? To what extent would an expectation of pretty-much-synchronous strong threading be breakable by NNTP?
(There's other fluff like identity management, which NNTP doesn't do at all, trusting whatever the poster puts in the From: field. But that's not part of the very nature of the problem.)
Even on single-system two-way gateways, the culture clash can be problematic. e.g. the wine-users list, which has a two-way gateway to the forum. No messages are lost, but the different posting styles clash badly.
So. Are there any useful suggestions? How to do forums over NNTP? How to map the user expectations of one to the user expectations of the other?
(Cheers to those providing useful suggestions and comments on Facebook and Twitter. Which will be the next frontiers.)