This an old thread, but what Will said in 2009 is still correct today, perhaps even more so since Penguin.
I fully agree with Will. We have done extensive testing on forum signature links and has only seen a minor ranking improvement when the forum topic is directly related to the site in the signature. There is zero effect for other sites.
Originally Posted by Will.Spencer
I have not seen any ranking improvements from blog comments for at least 4 or 5 years. By default WordPress adds the nofollow attribute to all comment links, unless the site owner has a dofollow plugin installed. Even then, we found no positive effects from blog commenting.
Google is ignoring or discounting these types of links. Anyone who joins a forum or adds comments to blogs just to pick up PageRank or improve their rankings is spinning their wheels. It is a waste of time and there is no SEO advantage.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin