I think I know what you are talking about, tabletroms uses it I believe, or was that notionink? Honestly I like the current chat system better. I hate having a bar at the bottom of the page, I never pay attention to it.
It would save on private message space. That way if you want to ***** at someone...uh I mean talk to someone about a post, you can work it out privately and there wont be any misunderstandings like in my 9/11 thread, where I was making fun terrorist holocaust deniers and not jewish people. Apparently I was misunderstood.
It would be usefull I think.