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