Take with a grain of salt, but the 64 bit kernel for Snow Leopard doesn’t boot by default on most consumer Macs at least in the developer pre-releases. That could change for the public release of course. Further, the 64 bit kernel won’t boot at all on Macs running an 32 bit EFI without a little Hackintosh trickery. The article is worth a read, but remember the caveats.
So far, it’s not yet known if Apple will enable owners of machines with 32bit EFI to boot into a 64bit kernel in Snow Leopard-final; Apple might offer an EFI update, or remove the artificial limitation. The same applies to MacBooks; it might be that Apple is planning on using 64bit as a selling point to drive people to buy MacBook Pros instead of ordinary MacBooks.
You can check your machine’s EFI by running the following command in the terminal:
ioreg -l -p IODeviceTree | grep firmware-abi