You leave me with no choice, double post time!
Here is the illustration:
Pretty straight forward. Taskbar to the right is the 'Multiple MSN Guy' concept. And the new idea is on the left...as you can see there is a ² beside the msn guy. That could symbolize any multi-user conversations, have the number change to match the number of users active in that conversation.
This can even be adapted in the original idea.
For example, lets say you are in a tabbed conversation with 3 people. The taskbar will show the name of the active contact, along with their status (as discussed), and a number as seen in the illustration to the left. In this example that number would be ³
Comments? Praise? Flames because of double post?