The new Apple Magic Mouse will go into standby mode if it is moved too far away from the computer to maintain it’s connection.  When the Magic Mouse is later moved close enough to the Mac to reconnect, it will not wake up until you actually click the mouse (moving it isn’t good enough).  This has been confusing many people posting to Apple’s discussion board, where they think the Magic Mouse is randomly disconnecting, but in actuality the mouse is just being moved too far away from the Mac.