Look in the source of any thread with a signature link in it, and look for the keyword of the link. You will undoubtedly find that it is not nofollow, hence, a dofollow link! Even if it does nothing for PR or SERPs, it will ultimately give you at least a couple of intrigued / interested visitors! Chuckun
I believe that Links are links, signature links are not excluded from that. Right now, Google have this mechanism of choosing which links makes value to a site or not. If the post w/ the signature link is posted in a relevant thread, I believe that it will give a value on it. That's my opinion.