I suppose, that if you set up a sub-domain with a different user than your regular domain, then should your forum software prove to have a security hole in it, you'd be better off. If it's the same user than there's no difference really.
However, as long as you keep your forum software up-to-date I rather doubt it's really an issue.
This is just my opinion, I'm no security expert or anything. You can't blame me