Wordpress founder, Matt Mullenweg, has a great feature in his photo galleries. In thumbnail view, any photo that has a comment attached displays the comment count under the thumbnail.
Look here (scroll down and you'll see some thumbnails with comment counts): http://ma.tt/2010/11/luxembourg/
He appears to be using the built-in gallery function, but I have been scouring the internet and cannot find any way to do this. Any help would be appreciated. Thanks!
EDIT: It should also be noted that in this particular case I actually don't need the standard captions to show at all. Instead of adding the comment-count to the caption, I would be happy with a way to replace the standard caption with the comment count. I'm not sure if this make things any easier. Thanks!