Hmmm...maybe you could include whatever forum script you having using PHP (or ASP, or whatever you want).
For example, the main forum (the only forum) could be at forum.something.com.
Then, blah1.com/forum, blah2.com/forum, blah3.com/forum, and so on, could use includes to pull forum.something.com in.
I'm not the person to talk to about programming, though. If that's what you're looking for (to have the same forum show on multiple sites) then includes might be the solution.
