Apple recently issued the golden master release of OS X 10.8 Mountain Lion to developers and testers, indicating that if no outstanding bugs are found in the current build then it will very likely be the final release of the operating system that will soon be issued to the public. Through the testing of the four preview releases of Mountain Lion, developers have noted that even though the operating system is 64-bit, Apple has excluded some early 64-bit-capable Mac systems from being included.
While this limitation was at first suggested as a possible temporary limitation during the initial phases of testing, Apple has maintained it through the development process and recently made it official on its Mountain Lion system requirements listings. On the Mountain Lion Web site, Apple specifies that the systems that will be able to run Mountain Lion are the following:
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)
You can see if your Mac qualifies by choosing About this Mac from the Apple menu, and then clicking the More Info button to launch the System Information utility. In the About this Mac summary window that appears, you should see your Mac's model name along with the time frame of its release, which you can compare with the above list.
The problems with the older systems that are not included in this list are twofold, and revolve around the fact that OS X 10.8 will be a 64-bit-only operating system that will not boot in 32-bit mode.
The first issue has to do with hardware requirements for booting to 64-bit mode. In some of Apple's earlier Intel-based Macs such as the first Mac Pro, even though they included 64-bit chips that can process 64-bit code, the systems' EFI firmware is 32-bit and therefore will only interface with a 32-bit kernel. As a result of this limitation, if a Mac system is only able to boot a 32-bit kernel, then it will not be able to load the 64-bit kernel in Mountain Lion.
To test this on any system, reboot with the 6 and 4 keys held down, which will force the system (if possible) to boot into 64-bit mode. Once logged in, open the System Profiler (or System Information) tool and select the Software section. In this section check the status of the 64-bit Kernel and Extensions listing, and if it says "No," then despite attempting to force the system into 64-bit mode, the system is not capable of doing so.
The second issue is a dependency on 32-bit kernel extensions. Current versions of OS X include both a 64-bit and 32-bit kernel, allowing the system to launch and load either, depending on its requirements and which kernel is currently being used. If the system requires kernel extensions (drivers) that are only available in 32-bit code, then it will only be able to load them when booted to the 32-bit kernel and therefore will not boot in 64-bit. While the system will only bypass loading third-party 32-bit kernel extensions when in 64-bit mode, if the kernel extension is a core requirement for OS X, then the inability to load it will lead to severely degraded performance, or a nonfunctional OS.
While one might argue that some Macs that are excluded from the requirements list ought to load the OS just fine, some of the kernel extensions that Apple uses to support hardware (particularly graphics) on these Macs is 32-bit only, and Apple has not dedicated the resources to bring these extensions to 64-bit.
Apple's reasoning here is rooted around the graphics requirements for OS X Mountain Lion. Apple has recently been making some advancements in its graphics support both in hardware with its Retina displays, and also in software with a push for OpenGL and OpenCL improvements and the inclusion of more graphics-intensive tasks and resources in the OS.
These software advancements surpass the capabilities of some of Apple's earlier graphics processors, such as the Intel GMA 950 that was included in a number of older Intel Macs. Since these graphics processors will not handle Apple's graphics developments in the new OS very well (if at all), Apple has halted support for them and therefore has not updated their drivers to be 64-bit capable. As a result, even though Mountain Lion could technically run on these systems, it would perform similarly to booting in Safe Mode with massive interface lag and other slowdowns.
Luckily as with its current support of Snow Leopard, Apple will likely maintain support for both it and Lion for a while after Mountain Lion is released, and therefore even though a few systems will not be able to run Mountain Lion, they will still receive security updates and other updates and continue to be quite usable. However, if you would like to take advantage of some of Mountain Lion's upcoming features, then you might consider upgrading your Mac.