Mochsa, you have one post so I'm gonna go easy on you here: I did not notice any increase until my 100th posting at DP. The forum siggys from DP factored into raising my Link Count. I also did the same at V7n.com, winamp, VoteForTheWorst.com, various blogs, auction sites with forums, classified sites with forums, Web portal sites with forums. Don't be a Debbie Downer until you have been a DP member and have 100 posts with a valid siggy link similar to mine!
I don't think it will be penalised for some reason, and there are over a numbers of known user in most of the forums had their sig links. and it is safe.
Google will not penailize you, but they will count the many links as just one. The side effect is that you could get direct traffic from visitors on the forum.