One of the limitations of mountain lion is that they are only going to support 64 bit Kernel extensions, the graphics drivers are are loaded as a kernel extension and some of the older macs (mac pro, iMac, MacBook, MacBook pro) only have a 32bit graphics driver (Kernel extension) avilable and so will not be supported by Mountain Lion.
To check just run this from your terminal (/Applications/Utilities/Terminal.app) :
ioreg -l -p IODeviceTree | grep firmware-abi
If it is supported the result should look some thing like:
| | "firmware-abi" = <"EFI64">
If you device is not supported you will get:
| | "firmware-abi" = <"EFI32">