This seems like something that doesn't really come up with. It only makes a small difference. Do you think that a forum with a Subdomain or folder would have better SEO and better pagerank? I use forum. subdomain but I just noticed everyone else uses forums. or /forums/ So I'm getting to wonder why they did that.
When you use a subdomain the search engines treat it as a separate site. So you don't benefit from links pointing to just www.domain.com. If you use a folder, the search engines treat it as part of the same site. As a result, those pages also carry some of the domain's strength. You can still rank well with either setup though. It just may take a bit more links to rank well for the subdomain.
No, at PubCon last year Matt Cutts came out and said google would be treating sub domains and sub directories in the same way. So your www.site.com and forum.site.com will be treated as the same site. Now there will be some exceptions with like blogspot of course, but you get the general idea now I hope.