I still have this issue on Linux Mint 14, which is a shame because I really like the cinnamon interface. So I have gone back to LXDE for the time being. I know it is not a ATI issue since LXDE has the CPU's Idleing.
I also don't think ATI is to blame since ATI atleast provides us the support and open documentation to the driver builders. Nvidia on the other hand gives us Linux users half ass drivers and no documentation or support in building open source drivers. I would rather have a company trying to help open source than stiff arming it. Nvidia has openly said that they will never provide full 3D support for Linux, that is a shame.
Back on topic, I will keep testing as cinnamon gets updated, eventually my main session will be Cinnamon, right?