I know we kicked around the idea of having some sort of IM system in place awhileeee ago, but the general consensus was that it would take away from the overall activity on the actual forums.
There are a number of outlets for instant messaging, things like skype, tinychat, etc. You have the option of turning on the notification where it gives you a pop up when you get a new notification, but I suppose you could always see the little blue notification thing at the top too.
Are you talking about more of a chat box/IM sort of thing, or something like facebook where it sort of updates the little notification number at the top when you get something new?