Atahualpa works flawlessly on WPmu (of course).
However, since WP's update cycles have traditionally been out of sync with bbpress, I'd be careful about integration - you may get it to work pretty easily the first time, but wait until the updates roll...-) - nonetheless, you will find some successful integrations once you start googling for them.
I'd rather go for a forum-like WP theme, or proper usage of the comment functionality, which kind of makes an extra forum superfluous - there are so many free and useful free storage offers - drop.io etc. - that you can really do without having contributors fill up your webspace... IYKWIM :-)
If, however, you still want one, there are a few WP plugins doing the job. The most elaborated one in terms of functionality is SimpleForum
IMHO, but I've scrapped it because I found it unacceptably slow at times, and ugly irrespective of the theme you are using. - A few years ago I tried RS Discuss, and found it pretty satisfying and conveniently fast, but it does not seem to have seen an update in quite a while.
Finally, if you want to go for the real deal (and ignore my above warning of upgrade nightmares), this
might be a good starting point to syncing "the big ones" (forums) with WP... enjoy!-)