Most of us are aware of the fact that top forums and blogs have included a "nofollow" attribute in their coding.By including this attribute "nofollow" in blogs and forums,the site owners ensure that the links added in the comments or signature does not affect the pagerank of the added link. In other words,these backlinks does not affect the pagerank of thier site.

Well,on a positive note,here is a best seo forum which explains how to develop on your links with sites having a "dofollow" attribute.Visit this discussion in forums.seo.com/showthread.php
?t=1792 for further details.