[dead7]

Why not just make the subdomains' documentroot the path to where your forum is now on the main domain? Then you don't have to move any physical files or update any paths.

You can then de-link it from the main website and just link to the new forum. Optionally you can still admin it from the same urls you always used. If you're worried about search engines you can 301 with mod_rewrite. Unless its a huge forum its probably in your best interest to just 404 the old site and let it re-index, 9 people out of 10 who attempt large rewrite schemes do it poorly. It may look good but they're lazy and do not check the actual server responses and make sure that every possible combo of url scheme works within the rewrite. This leads to duplicate content and bad indexing. Unless you're 100% confident that what you're doing is perfect, just let it reindex. If the site is fairly established with in bound links it won't take that long to get reindexed and you can be assured you won't get a defunct indexing like a bad site-wide 301 would give you.

Then just update your robots.txt with Disallow: /forum (if thats where they are now on the main TLD.)