Notify users in the UI about new messages from the activity stream

One of our users came up with the following suggestion:

It would be nice if one could have a small notification popup on the wiki pages telling a user if there are new messages in the activity stream. (I have been told facebook has something like that.)

I am aware that there is no data structure to mark messages (or other elements of the activity stream) as read/unread by users, but maybe something like a "X new messages since last view of activity stream" is good enough - in that case one only need to remember the last display of the activity stream, either by a timestamp in the user profile, or even just in cookie.

Probably a relatively basic implementation could be made with a UI Page with one JavaScript extension that queries the activity stream state on page load for the last events since Time X and inserts the results somewhere and a backend Json Service that provides the necessary data.

Attached a very loose idea what such a notification info might look like; positioning and layout of the notification is only to illustrate the idea and can certainly be improved. In the simplest case this notification info would be a link to the users profile page, which shows the most recent activities (beside other stuff).