Most of the people think that forums are useless and building links from them does not affect rankings. Forums signature can be useful only if they are from PR page, I have noticed Google or any other search engine gives very less value to signatures. Have you ever tried to check your backlinks? You will notice that none of forum signature will be showed up in results. However, forum signatures do counted as backlinks but only useful when thread gets older and it gets some PR. I have seen that if you place your link in post not in signature, search engines will count as good back link. Sometimes No Follow links are shown in Google results as a backlink when I placed the link in posts. Recently I was assigned to do 1000 forum posting on high PR forums. I have learned many things from that project; we can target the keywords through forum signatures. My client was asking to place the link on those pages which have gone through Google last PR update and it worked out pretty well. When I made posts on those pages they already have some PR3 to 4, some of the pages have low PR but it is also useful. When I completed the 500 forums postings with signature links I have seen the keyword I was targeting comes up in Google search on first page after 1 month.