In the newest leaked build of Snow Leopard, 64-bit booting isn't enabled by default on certain older (though not that old) Macs, which instead boot automatically into 32-bit. The question: Is Apple backtracking on Snow Leopard's 64-bit capabilities?
As it stands right now, with the current build (10a432), the answer ranges somewhere between "sort of" and "no". Essentially, older Macs that are actually fully capable of booting in 64-bit (like some of the earlier Core 2 Duo-based MacBooks) will by default boot into 32-bit. However, you can force 64-bit booting by holding the 6 and 4 keys while starting up, and even if you don't, those machines will still be able to run 64-bit programs without trouble. So the restriction from automatic 64-bit booting is an intentional move from Apple.
Update: We've gotten some explanation from Apple on the issue at hand. The deal is, Mac desktops and laptops will indeed boot with a 32-bit kernel, while only the Xserve will be 64-bit. But this won't matter to the consumer. Our Apple contact explained:
The 32-bit kernel fully supports 64-bit applications, all system libraries that 64-bit applications use are fully 64-bit, and 64-bit applications have a full 64-bit virtual address space of 16 exabytes available to them on Mac OS X. The primary benefit of a 64-bit kernel is to improve the efficiency of accessing over 32GB of RAM.
As a whole, machines with lots of RAM (over 4GB) are more efficient with a 64-bit kernel, but even though consumer Macs will have a 32-bit kernel, all Intel-based Macs will run 64-bit programs automatically, which gives you the most obvious benefits. So really the only benefit to having a 64-bit kernel is if you've got 32GB of memory, a frankly monstrous amount of RAM, and hell, the hardware won't even allow that much. All 64-bit applications will run on the Macs with either Intel Xeon or Intel Core 2 Duo processors, so this 32-bit brouhaha is pretty much a misunderstanding. [Ars Technica and OS News and MacWorld]