Mountain Lion Drops Support for Older Macs
So as with iOS, Mountain Lion drops some older hardware.......
Here is the fully classified list of devices compliant with Mountain Lion, which will be arriving later this month for the upgrade price of $19.99.
iMac (Mid 2007 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)
To see if yours is compliant type the following command into Terminal.
ioreg -l -p IODeviceTree | grep firmware-abi
All unsupported Macs will return EFI32, if you have "EFI64" your good to go.