Hello. Here is my xnu patch. Tested on i386-pc, i386-efi and x86_64-efi. 
On non-efi it needs efiemu otherwise you need only 
include/grub/autoefi.h file from efiemu patch. To resume xnu from 
hibernation do:
xnu_resume <hibernation file>
Note: you don't need efiemu in this case
How to boot xnu:
<efiemu if not on efi platform>
[on pc only:] vbe_mode=0xYYY # desired vga mode
fsb=133.3 # your fsb frequency
xnu_kernel <kernel> <command line>
<insert modules>
boot
Modules can be inserted one of the following ways:
1) xnu_mkext <mkext file>
2) xnu_kext extension.kext
3) xnu_kext extension.kext/Info.plist extension.kext/extension
4) xnu_kextdir <directory containing extensions>
It's also possible to execute these commands multiple times
The most typical case is
xnu_kernel /mach_kernel rd=disk0sX
xnu_mkext /System/Library/Extensions.mkext


If you need to add values to device tree the command
xnu_devtree <devtree file>
This file uses the following format:
    valuename:valuedata;
    keyname{
      contents
    }
    keyname, valuename and valuedata are in hex.

If you need to adda ramdisk execute
xnu_ramdisk <ramdisk file>
ramdisk will be exposed as /dev/md0 which may be used as boot device 
with rd=md0.

The areas which need more work (every help is welcome):
1) testing on different platforms
2) Detect "device-properties" value in device tree. There are several 
trivial values in device tree present in boot.efi but not in grub2. 
These ones are easy to add and AFAIK don't change anything. But the 
value "device-properties" is difficult. I already know it's format but 
not where the values come from. It contains info about gfx and sound 
card which may not work if this value is missing. The current workaround 
is xnu_devtree command with a dump of device-properties
3) autodetect fsb frequency
4) Support video splash
5) Define and use an unified interface to retrieve video information 
uniformly across platforms
6) Boot by UUID. I know how to do it but need md5 for it which is a part 
of pending luks patch
7) Scripts for automatic creating of grub.cfg entries
8) Support for prelinked kernel. It's compressed and I have yet looked 
how to decompress it (seems it's compressed with something called lzss)
9) Use claimmap once available (see my multiboot on efi patch)
10) Better collaboration with memory management once advanced mm's available
11) Resume from encrypted hibernation

-- 

Regards
Vladimir 'phcoder' Serbinenko