When I read an emailed Notification about a Comment, it should mark the Comment as Read

(User Story, Investigate , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Conversations, Email, Notifications
Summary: It annoys me that few forum systems do this: until you actually go to the site, it remains unread, despite the fact that I have read the message.
In theory, this seems straightforward to implement: in the email, embed a 1x1 pixel white image whose URL encodes the comment in this email. When the image is "fetched", we immediately return a standard 1-pixel image, but also send a message saying that the comment has been read.
Ideally, users should be able to switch this on and off.
This probably won't work if the user is reading offline, or if they have disabled remote images in email. But it seems like it would work in most cases, and would be convenient.