Apple has now confirmed via its Mountain Lion upgrade page that previously published limitations on some 64-bit Macs will extend to the final release. This means that several otherwise 64-bit capable MacBook Pros, iMacs, and Mac Pros will indeed be restricted from upgrading to OS X 10.8 when it goes public later this month. And according to information found in the recently released golden master (GM), the limitation appears to be related to graphics, as we originally suspected.
When the first developer preview of Mountain Lion was seeded to developers earlier this year, the release notes listed hardware requirements showing that some early 64-bit Mac models were not compatible. (Lion is likewise 64-bit, and can run on any Core2 or newer 64-bit Intel processor.) As such, Mountain Lion developer previews would not run on the earliest Mac Pros, MacBook Pros, iMacs, and other hardware.