I agree with louie 'every little helps' tagline. It is a little benefit I believe, particularly in the small irish market where other competitors might not have any keyword rich backlinks.
Can Google tell if a forum is a forum? Boards that use modrewrite to turn dynamic pages into static .html would give the best benefit I imagine. I would like to see vbseo include a bit of logic whereby a persons signature could be alernatied between two different ones, to make links from a single forum/domain look a bit more natural.