Yes it definitely helps, but it is not like something you will rely upon as your main source of traffic...
Darn, too bad most DP members have pop-up blockers or you could make a small fortune before the word spread and everyone hated you.
I think sig links can be a very effective advertising tool and can be very beneficial if you multiply the amount of times your sig is seen by hiring people. I mean $5-$10 bucks for someone to make 300+ posts over a month. Certainly something that can be profitable given the right sig and right product or service. If anyone's interested a good site for buying and selling sigs is www.sigtrader.com.
Signature links do bring in lots of traffic especially if it is catchy and generate interest from people who see it.