And the GPUs in some of those early 64-bit Macs were deprecated before 64-bit KEXTs became common. Since those older drivers are 32-bit, Mountain Lion won’t load them. We believe Apple decided it was better to draw the line in the sand for some older machines rather than invest the resources into updating the drivers for these older GPUs.
Some Macs from as late as 2009 will not be able to run OS X 10.8. This is more of an issue than it might otherwise be because of the Mac App Store. Apple requires that new applications and updates be sandboxed, but many of the sandbox features are not implemented until 10.8. Thus, developers will be forced to drop support for 10.6 and 10.7. If you choose not to update to Mountain Lion, or have an older Mac that can’t run it, you won’t be able to get updates to your apps, nor will you be able to re-download the compatible version that you already purchased.