I think it would be nice if we could turn it on or off at our own discrection. That way a person could be online but be a little descrete (like if you wanted to read the forums but didn't want to move, or something similar). I also think this would be good because of personal experience in which I have been logged on and very attentive, but none of my opponents have moved recently. I may want to start a new game, but the lack of a green dot at the time doesn't reflect the actual amount of attention that I am giving the site at the moment.
I think it would be good if the " green dot" was another function from the My Home page, with personal settings and preferences for it and such. If it ever were to be a personal option, it could be done so that instead of it coming on for x amount of time after a move, it could be turned on from the My Home page with an automatic turn off time that could also be set from personal prefences and settings. I believe that this would allow for maximum flexibility and personal enjoyment of the feature.
However the green dot ends up being done, I am very happy that we have it now. Three cheers for Russ!