Users who want to run the new 64-bit kernel on late-model Macs (pretty much anything released after early 2008) can do so by booting with the 6 and 4 keys held down. If you're wondering whether your Mac has a 64-bit EFI firmware, you can type this command in Terminal:
ioreg -l -p IODeviceTree | grep firmware-abi
The response will identify the machine as either having 32-bit or 64-bit EFI. I found this tidbit in this article on AppleInsider.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090902100235176