I've spent the last 4 hours looking for just this and have come up empty handed.
I want something that allows users to add annotations to photos that I post and have the annotations show up as a comment as well so others can reply etc.
Basically I'm looking for something similar to dannychoo.com. Which is mainly a photoblog with comments and user submitted posts. All photos posted can have notes (annotations) added as a comment.
This is not something that WordPress has and I'm already looking for another blog solution before considering just making my own.
Good luck on your search, but from what I've gathered this is not a much sought after WP feature or it's too difficult to integrate into the current system. Otherwise it would have been made already.