If the current developer seed (build 10A432) of Snow Leopard is indeed the Golden Master version (as has been highly rumored) only Xserves will be able to boot into 64-bit version of Mac OS 10.6 by default. All other Mac users will have to hold down the "6" and "4" keys at boot to load the 64-bit kernel and kexts. Every time.
OS News reports that some Macs with 64-bit processors won't be able to load the 64-bit kernel because they have a 32-bit EFI. The seed notes explain which Macs can boot into a 64-bit kernel and drivers by default (Xserve), and which ones are only "capable" -- meaning you have to hold down the 6 and 4 keys at boot.
Here's the list:
Still not sure if your Mac has the 32 or 64-bit EFI? You can check your machine's it by entering the following command in Terminal:
ioreg -l -p IODeviceTree | grep firmware-abi
OS News notes that only Macs with a 64-bit EFI are able to boot the 64-bit Snow Leopard kernel and kexts; an artificial limitation imposed by Apple, even though a 32-bit EFI can boot a 64-bit kernel "just fine."
What's worse is that even if your MacBook (non-Pro) has a 64-bit EFI, it will only be able to boot the 32-bit version of Snow Leopard because of a limitation that Apple imposes on MacBooks. The biggest roadkill on Route 64 is the original Mac Pro (which was discontinued January 8, 2008) it won't be able to boot the 64-bit kernel and drivers either.
If you're worried about being stuck in the slow lane with a 32-bit EFI, there's hope. The Netkas blog has posted some workarounds for booting the x86_64 kernel on a 32-bit Mac. Also, keep in mind that it hasn't been confirmed that 10A432 is the GM version of Snow Leopard and Apple could change the limitations above any time via an EFI update.