For the past year or more, Google's Penguin updates have focused on artificial links such as forum signature links and other links that webmasters create for themselves one way or another in forums, blogs, online directories, etc. - especially when done in large numbers. I would not recommend posting in forums just to get a signature link, but if you were to participate in small number of forums that were related to your site and occasionally include a relevant link to your own site in the body of your message, you'd probably be OK and the links might even help your rankings a bit. But you definitely want to avoid having your rankings reduced by Penguin. Very few people have managed to recover from it.