But does that really matter? Whilst PageRank is part of most search engines algorithms it is hardly significant. It's also pretty easy to manipulate with a tool called a 'wallet'. Google is looking for real value for its searchers, not just sites with the deepest pockets (hey, that's what adwords is for). Forums give you the oppurtunity to drive highly targeted traffic and also seed your own backlinks with appropriote anchor text. Even if it is 'no-follow', search engines will still evaluate the anchor text, origin and destination of the link - maybe taking into account that it is a forum but none-the-less there is some value there.
This question gets asked a number of times and you always get different answers. The fact is though that everyone giving those answers is on a forum and has sig links at the bottom of their page. That should tell you that everyone thinks they are worth having.
Regular posting on forums not only helps in bringing in traffic to your site but also helps in building backlinks (from dofollow forums) for your site. Make sure you follow the forum rules and not spam the forum just for the sake of building links. SticKer