Sounds good SB - but it would be even better if you could display different forums for each site
and perhaps just use the user table. As surely once you give them their 'own' forum they will want to control things add forums, not display forums that are not 100% relevant etc.
If you want to include sites that are not hosted on your server - then why not set up forums.theirsite.co.uk on your server, whilst
www.theirsite will remain on theirs - it will need a little tweaking with DNS, but it should work fine.
at the moment - but expanding all the time....) 
