I just saw that Tax Almanac, a MediaWiki customized by Intuit software, has a very interesting integrated discussion system:
http://www.taxalmanac.org/index.php/Discussion_Forum_Index
It uses normal wiki pages in the "Discussion:" namespace to store the threads, and stores the comments themselves as parametrized templates within these pages. But the user interface looks like any ordinary bulletin board.
I don't see any links to source code and don't expect that it is available. Has someone talked to them about whether they'd be willling to open source it already?
Erik