Apple sunsets a few 64-bit Macs with Mountain Lion, video drivers likely the culprit
Apple is well-known for wanting a close spread in hardware requirements with OS X upgrades, having dropped PowerPC like a hot potato when Snow Leopard arrived just three years after the Intel switch. Whether or not you're a fan of that policy, it's certainly carrying forward with Mountain Lion. When the newly-finished OS hits the Mac App Store, it will rule out the very first wave of 64-bit Macs: certain MacBook Pros, Mac Pros and other early systems will be denied a taste of 10.8. Some sleuthing from Ars Technica suggests that it's a matter of graphics drivers rather than capriciousness on Apple's part, as the Macs excluded from the mix are using 32-bit drivers that won't play nicely with Mountain Lion's 64-bit Utopia short of a wide-scale conversion effort. It's little consolation to those who dropped a pretty penny on certain Macs just a few years ago. That said, Apple is still going the extra mile to support some systems -- if you're reading this on an original aluminum iMac, you're sitting pretty.
