Make sure you place the keywords you want for your link and you will see a small rise in serps. The main reason for forums though is learning/teaching rather than building links.
The mighty backlink my friend is what makes it significant. Some here are even selling their signature links for that very reason.
Yes i agree, the backlinks that you get from forum posting is just a bonus while learning the things you come to the forum for. The backlinks might be weak but if you have a lot of them they still can make a nice change to your serp rankings.
yes definately forum signature is beneficial.As it helps to get qulity backlink. Also through signature other get short idea aout your niche which can give you littile traffic. And signature is sort of your identity.
About a month ago I added a signature link to the new website of someone I know (sig link #1). In about one month that link has generated over 600 backlinks to his site in Yahoo! Site Explorer. Now whether those sig links count for much, that's another story, and I don't know the answer to that.
Seek out dofollow forums and post on them. That way every time you post you earn a low weight backlink.