[@sadr Participant 15 years, 3 months ago]

Maybe it’ll be different in the future, but right now you can achieve that by creating a sub-blog called forum at forum.mysite.mydomain, then you install bbPress 2.0, network enable it and enable it on the ‘forum’ blog. Just find/make a really minimalistic theme if you want to focus on the forum itself. With a little bit of tweaking you could pretty much do away with all the Blog/CMS elements and be left with just a clean forum.