Gmail fixes "mark as unread" annoyance - you can now mark individual messages instead of the whole thread

Great to see that Google have fixed that issue where "Mark as unread"
marks *the whole thread* instead of just the specific message that you
wanted to mark as unread. I'm just surprised it took so long to get
it fixed.

http://gmailblog.blogspot.com/2009/03/small-but-helpful-change-to-mark-as.html