Show Private Messages as "seen" when the other person view's it

There should be an option to show if a message has been read or not. This will allow community members to have an idea if they are ignored or not. Other scenario's tie into this as well. Even phpbb2 back in the day had this feature, facebook however came in and made it a bit neater in the inbox by not forcing the user to look through two different parts of the inbox to see this message. It's a small addition, but certain a nice one.