couldnt you have both a forums.site.com and a site.com/forums and have the /forums redirect to the subdomain?
another consideration: it's much easier to sell links on a subdomain than on a directory, even if they're the same PR don't ask me why...
I suggest you to go for sub domain and then register additional domains for the forum and then point that domain to the folder forum.
i would suggest you to go for subdomain because if your forum is a bit differnt from main website it can get listed in seach engines as a differnt website