Whether your Mac's CPU is 64-bit or not, Mac OS X 10.6 Snow Leopard's 64-bit kernel is normally only able to load if the machine also has a 64-bit EFI. You can tell if your machine has a 32-bit or 64-bit EFI by typing this command in Terminal:
ioreg -l -p IODeviceTree | grep firmware-abi
However, according to OSnews, the OS X seed notes claim that the 64-bit kernel of Snow Leopard is not supported on any 13" Mac laptop, even if it has a 64-bit EFI. Despite this, 64-bit Snow Leopard loads on my 13" MacBook Pro (aka MacBookPro5,5) just fine.
The latest 13" Mac lives up to its "Pro" name after all.
Despite having this 64-bit capability, I will usually run in 32-bit mode since 32-bit kernel extensions (like printer drivers, etc.) won't run when the 64-bit OS X kernel is loaded, and 64-bit applications can still run normally with the 32-bit OS X kernel.