I've just spent the past few days playing around with bbPress and the GetShopped support forum plugins (and bootstrap!).
I've just launched a little support forum here: http://ultimateclientmanager.com/forums/
Still trying to figure out the best way to integrate Envato Purchase Code checking. I think I'll put it on the registration form, and another option in the sidebar so people can add multiple licence codes down the track.
bbPress user registration was an absolute nightmare to get working on multisite - maybe it was just me - but it was so hard! still not 100% sure it's working right.