There are few who argue that Google devalues links from forums or doesn’t think highly of them; however what about links from thread which are PR2, 3? Is that equivalent to a PR 2, 3 links from a site or a directory?
There are a few things to consider: 1. Some forums automatically nofollow all links in posts and/or signatures. That obviously means they're of no value. 2. A typical thread page in a forum has a LOT of links. You'll get some benefit from yours, but it's just a small slice. Look at the header links, breadcrumb, other posts on the page, links to profiles, leave messages, etc. Bottom line: They'll usually help, but not very much.
I for once agree with another poster in a thread! The above thread basicallty is everything I would of said. You should note that a properly SEO'd forum would use nofollow to profiles, private messages, etc.