I think you can try out forum as service. We've been servicing our clients for 2 years and had faced need of this. Well integrated under custom domain, white labeled and customized forums are something that I won't mind paying for.
In which case, Disqus.
Or just comments on things?
In which case, Disqus.
Also, if you're doing forums as SaaS you really need to think about the kind of anti-spam, moderation and admin tools that you're going to provide.
I don't know if you've ever seen the admin section of vBulletin, but there's a reason why vBulletin is so widely used and it's not to do with the front-end.