I was really surprised this hasn't been implemented and I can't find any plugins or hacks that will do this. If a user is logged, when they go to leave a comment all the information should be filled in for them (name, email, uri (assuming they entered one in their profile). And then at a time later down the road link this with the publicly viewable profiles that another person requested. So it will say the comment is from joeuser and you click on that and it takes you to their public profile page. Plus it would be more encouraging for people to self-register if that happened as well.