Yes it does help as a backlink towards your site. Some forums have rel=nofollow attribute in their signature areas so that kind of links will not do any good.
Sorry mate, but before replying to posts read them throughly, please. I wasn't saying that google treats ALL links from ALL sources equally. I said that ANY link visible for Google's robots has SOME value (except of links from linkfarms and other places considered "illegal"), therefore links from forum signatures DO have some value that is definitely HIGHER than 0.
forums signature help to get traffic, if you can see some threads have pr1 or pr2. so backlink count.
Some very suspicious friends of mine say that google can track optimized sites this way, so they never put links of their sites in signatures of webmaster forums
Sometime, forum signature is a good place to "attract" search engine spiders. It is just a another method of submitting sitemap to www.google.com/webmasters/sitemaps
Even if it didn't help on your Serps, it is still benefical as forums are a good place to ask questions, get help, answer questions, etc. about your sites topic and why not just place a link in your sig and get some targeted traffic. I find that my traffic from DP is more targeted than traffic from Google.