Signatures on DP are without rel no follow, so they are counted by the Search Engines. But whether they weigh in much towards trust for a website is another issue.
I've seen somewhere that it is not a good thing to get thousands of URL pointing to your site with the same anchor text. What do you think about this? I see people here with thousands of post (thousands of links) with the same anchor. It doesn't seem to affect anyone.
Well any traffic that I get from my sig in the few forums that I visit regularly, rarely does the traffic lead to sales, but if it leads to someone linking to me from their site, then it's worth it. Ya know, viral marketing and all that happy crap.
Google will recognize a forum when it sees one. And signature links have been around for ages. So I don't think there is anything to worry about. Remember: anything a person might do who knows nothing of SEO ought to be harmless.