I've been testing this method for awhile to determine if it was possible to boot the x86_64 kernel on the Macbook Air via any process. It looks like it is.

References here and here show it is possible to force boot the x86_64 kernel if you have a 64bit EFI firmware.

Theory is fine, but for now I haven't been successful.

There are several relevant parts such as:

Model (with 64-bit EFI) Byte Position in boot64.efi Old Value New Value
Mac mini(Eary 2009) 0266DB 000 080
MacBook Air(Late 2008) 0266FB 000 080
iMac(early 2008) 02671B 008 080
included in the comments at this page, but so far I'm stuck.

As an alternate method I attempted to use lipo to modify the mach_kernel to only contain the x86_64 kernel information. A reboot confirms this didn't work so well.

Interesting notes so far from the EFI modification tests:

1) bless --info works fine on 10.6.0, but gives a Can't access "efi-boot-device" NVRAM variable on 10.6.2.

2) You can still modify which efi file is used for boot in 10.6.2 (I fouled one up in a hex editor and rebooted to see if it would fail just to test it), but the bless binary will not show the information the system is configured with.

3) The information at both links above for modifying the boot.efi doesn't seem to work in 10.6.0. It already wasn't working in 10.6.2, but there were some comments about it working in previous versions of Snow Leopard so I went back to an unpatched vanilla install from my install media (used Disk Utility to format the drive before installation) to test it.

Does anyone have experience with this?