This is a commonly used technique in game alphas under NDAs (TESO and WoW WLK are the most recent example that come to my mind). The player's account information are embedded in the exif of the screenshots. If a screenshot leaks from someone who doesn't know better, it's possible to track it down.

I've though about it, but there's a privacy concern that turns me off, it's the equivalent of having exif data in pictures, it's really great, until someone decides to do something questionable with the embedded data.