I had no idea you could do this until now! Does your WP7 app have a bad review, and you wish you could get in touch with the person who left the review? (Not to harass the person! To get more info, fix the problem, and turn them into a happy customer.)
In Zune, click on social, then INBOX, then the teeny-tiny NEW MESSAGE button at the bottom of the screen. You can send them a message using their screen name shown in the review. And it’s better than Twitter: You can use up to 230 characters!
Unfortunately, the recipient’s new-message notification is very subtle. (Some text appears under their name in the top-right corner.) And messages expire after 30 days. Still, it’s much better than nothing!