TweetBacks WordPress plugin & WordPress › Twitter Comments - Twitoaster WordPress Plugins does it, sorta. It integrates it into the wordpress comment box but does not show them separately. The reason I need to show it separately is because I don't use the traditional WP comment box, I use a Facebook comment box.
It doesn't have to be inserting as a "comment", it could just be code that will display all tweets that mention my posts. I would also like to do the same for Facebook.