These links are the one's that got me thinking.

http://www.mattcutts.com/blog/subdomains-and-subdirectories/

http://www.webmasterworld.com/google/4339166.htm

Also, I was under the impression that sub forums on one domain were viewed as different sites in the SERPs but try searching for 'bodybuilding'.  You'll see www.bodybuilding.com at the top and a list of pages from that site and 2 further subdirectories are included (forum.bodybuilding.com and bodyspace.bodybuilding.com).

Although a personal preference only, I prefer subdomains and think forum.domain.com and blog.domain.com look much better than domain.com/forum and I would do it this way IF the rankings were unaffected.