It would actually be nice to have a list of computers from Apple that DO, do 64bit.
I'm uncertain if my Santa Rosa MacBook Pro supports 64 and my girlfriends White MacBook white (2007) does as well.
The 64-bit kernel can be tested on the following configurations:
Machine > Model name > K64 status
Early 2008 Mac Pro > MacPro3,1 > Capable
Early 2008 Xserve > Xserve2,1 > Default
MacBook Pro 15/17 > MacBookPro4,1 > Capable
iMac > iMac8,1 > Capable
UniBody MacBook Pro 15 > MacBookPro5,1 > Capable
UniBody MacBook Pro 17 > MacBookPro5,2 > Capable
Mac Pro > MacPro4,1 > Capable
iMac > iMac9,1 > Capable
Early 2009 Xserve > Xserve3,1 > Default
Using this seed*and one of the K64-capable machines listed above, simply boot the Mac with the '6' and '4' keys held down to use the 64-bit kernel. Observe that*uname -v*reports*RELEASE_X86_64. *Machines listed as "Default" and all Server installs will run K64 automatically when loaded with*10A402.
You can also set*arch=x86_64*in your*boot-args*NVRAM variable, using*nvram(8). When you're done, you can remove the boot-arg, or if you can no longer boot into an OS to unset it, hold command-option-P-R to zap NVRAM.
If you just want one partition to boot x86_64, edit the file /Library/Preferences/SystemConfiguration/com.apple.Boot.plist and add*arch=x86_64*to the kernel flags.
If some functionality is not working and you must revert to using the 32-bit kernel, you can either reboot with the '3' and '2' keys held down or set*arch=i386*in your boot-args.
... Hope this helps