This one was suggested a long time ago... Takes me back. The problem is that it is difficult to determine whether people are actually reading or just have a window open. And due to the way cookies may be handled, it is possible for people to simply never log off.
I think it's not worth the trouble, and even if it was implemented, it would be unreliable. I could set Opera to refresh the main page every few minutes, to simulate full days of enlightened discussion...
