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/Peo, FreeWebSpace.net
If you have some server administration skills it shouldn't be too hard to setup manually. When a user makes a forum simply add a virtual host to apache, make and dump a database for the forum software, and make a copy of the forum. Then you'd simply have an automated forum creator at which point you could code in your own user management system and what not.
Not sure though, I haven't made a multi-forum host before. :3
I have done my research a few months ago before I was considering to start my own host, without any luck. The only paid solution would have costed me ~$80 and judging from the fact that the developer replied to my simple question nearly 2 weeks later.
I am unfortunately unable to post links and I would rather not try to trick the system... if you google "MyBBForumhoster" you will find a pretty good script. You could offer not only sub-domains but multiple main domains to chose from. It is also possible to let your webmasters use their custom domain.
Feel free to message me and we can make a better deal than the one listed on the website. I am providing an alternative, hopefully relevant to the OPs interest.