I know that in YouTube and vBulletin alike, you have this little scroll down bar where it says a 1 or a 2, depending how many "new" or unread things that go on on your profile.
For examples, if you have a subscriber, it would say a 1 next to your name to indicate you have a new one.
If someone posts something on your wall, there would be a 1 next to your name.
If someone PMs you, there would be a 1 next to you name.
This has already been implemented into AG3. You get notifications when people inbox you or send you something on Beta. But instead of the number next to your name, it's on the Activity Stream.
