Comment on WordPress Gravatars by Mark.
I saw yet another recent comments with Gravatars plugin that had a feature that if someone does not have a gravatar it has a little suggestion to sign up and a link to gravatar.com. It has a question mark with a shaded icon to the left of the comment information form (name, mail, website, comment title).
It is based on the idea many people do not know what a gravatar is and need a nudge to sign up. It could open the link in a new window and be no follow (just kidding about the no follow, it would be part of the form).
There could also be a choice of displaying your Facebook icon also. I think this makes the Gravatar plugin a more complex project but I thought I would put it out there for back burner ideas.
I tried hacking a few of these things myself, but I see for it to be perfect, it needs more work.