I installed Snow Leopard on a Macbook Pro (2.4 GHz core 2 duo "penryn"), and was surprised to find that it was running the 32 bit kernel despite the machine being 64-bit capable...
uname -a showed the i386 kernel was running
I really wanted to try the full 64 bit kernel and there exists no force64 kernel flag...After a bit of hacking I've figured out how to force 64-bit boot with Snow Leopard...Full instructions at
osholes.blogspot.com //my blog
Basically I removed the i386 kernel from the universal binary, causing it to select the remaining x86_64 kernel (can't choose PPC) and run it. Note that no 32-bit kexts work.
good luck

man lipo