I will let
Apple Insider answer that for you.
Caution: It drives my wife crazy when my iPhone, iPad, and Mac all ring at the same time.
About the all ringing at the same time—I know joemikeb runs the latest of the beta's with all of his OS's, but there was a change in a recent commercial release of OS's so that rings only happen on one device. I don't know how to choose the device (perhaps the one actively in use), but I don't get multiple ringing when I'm on my iMac.
