Luke, from my personal point of view, signature links and blog posts ( if they are DoFollow ) is good for link building purposes and blog commenting on related niche ( webmaster blogs, make money blogs etc etc, no matter NoFollow tag applied or not ) is good for branding purposes. Btw, content is the king.
yeah absolutely.. Signature Just make some quality post to create some buzz in the forum, everyone visit your thread and you get the exposure..
Also try out to get registered in forum directories like this; http://directory.big-boards.com/ More list here; http://avinashseo.blogspot.com/2007/09/forum-directory-list-with-pr.html