I'm going to be honest here, I can't stand the signatures on most forums, they get way out of hand too quickly with giant flashing images or masses of random annoying pointless text. It gets in the way of the content.
At the moment I think some users sign their name after their answers, which is fine in small doses but don't go nuts with it.
Speed freak is actually right here, the avatar and username is your signature. I may look into expanding that a little in the future though.