Apple has confirmed that some 64-bit Macs will not be able to update to OS X Mountain Lion. The limitation appears to be related to graphics, but Apple didn't share any details on why these Macs are no longer supported.
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.
Apple declined to tell us the reasoning behind leaving some of these models out of potential Mountain Lion upgrades, but we suspected it is related to an updated graphics architecture that is designed to improve OS X's graphics subsystem going forward. Our own Andrew Cunningham suspects the issue is more specifically related to graphics drivers, since the GPUs not supported under Mountain Lion have drivers that were written before 64-bit support was common.
Macs supported by Mountain Lion:
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)