A lot of forums use the nofollow as part of their href tag in order to deter link spammers.  The nofollow can be used either on links that appear in the message body, links that appear in the signature, or both.  I participate in a lot of forums and although it is usually the content and community that make me join, stay and participate at any given forum, I would also join dofollow forums in my niches with less traffic and activity.  Perhaps we should start a thread here listing dofollow forums by niche?