Probably a spare USB slot. http://www.engadget.com/2008/01/31/logitechs-mx-revolution-goes-bluetooth-at-the-fcc/
https://forums.macrumors.com/posts/4941087/
Yes, it is very comfortable. It's a lot lighter than the MX1000. When I go back to it, it feels like a brick. The MX Revo is that light you can control it with two fingers, without any strain on your wrist.
Nice, I'm picking one up from Circut city tomorrow for $69. Logitech Bluetooth sucks because it's propriatary you need their dongle, so it still takes up USB slot - not that I need anymore. It's also slow and laggy.
I can't wait to try it out, hopefully it will feel better then the mighty mouse! Does it hold a charge fairly long?