There are many forums with different PR but signatures still don't carry that much weight because every sig link that points to your site all originates from the same domain (forum).
Forum signature is one of the traditional means to promote website. People were using that before blogs and social bookmarking sites appeared. Dofollow signatures still carries some weight today, but probably less weight than articles marketing, assuming the forum and article directory have the same PR.
Does 1 Signature from a domain carry the same weight as 100 from that domain (ie: 100 posts)? Many thanks for all of your input!
not much, mainly because there is a good number of outgoing links from almost all forum pages. however being on a relevant page should help better.
well signature links holds lots of weightage. Ex: Suppose if a user has 500 posts in a forum and puts a signature link.Then it will be counted as 500 links by SE.And regarding too many links from same domain name doesn't help,well thats a myth.