Ive heard that SEs like links from forums bc they are updated constantly etc. But most fourm pages arent even indexed, so how would links from those pages help?
See: Do forum message body links help PageRank and SERPS? Do forum signature links help PageRank and SERPS? But don't take my word for it, run the tests yourself.
i've seen a few of my links in forums register but some don't, just watch out for nofollow codes in some forums i guess.
i think Google pays very less importance on forum links. IMO, they are just good to get you some traffic.
I've had administered a small forum and today, 6 months after it closed, it's PR4. No links other than other forums. Good Luck!