Well as a side effect of current research I foudn following procedure to boot AppleTV OS on any MAC (in whole, without a OSX install below)

- copy OS.dmg from the recovery parition of your AppleTV
- use diskutil to mirror it to any parition you like (prefered disk0s3)
- replace boot.efi in /System/Libary/Coreservices/ with the one from your OSX install
- put any 4.8 or 4.9 mach_kernel into / of the drive
- copy over your Extensions from your OSX to AppleTV OS (/System/Libary/Extensions)
- replace Finder with the Finder with patches
- bless boot.efi

...done

this setup is much slackier than installing a whole OSX to put the Finder with patches in.

Have fun

Edit: I forgot something, remove the cache entry for kernel from com.apple.Boot.plist! IMPORTANT!

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Great and easy way to do it .... only thing left to get working now is kb and mouse support on hacks and my mac

I tried doing this with my 1 GB SD card /w reader and it didn't work

Is the OS.dmg ~200 MB? if so, then that is the one I'm using. Also, it lists problems with launchd during verbose mode. Do you have any ideas?

Could you post a slightly more in-depth howto?

I know how to do some of the steps, but not others...

Some terminal command listings would be nice maybe? i figured out everything but the mach_kernel thing and the kernel cache.

And does this mean we get that kickass intro video?

--neg

Just an FYI; I'm fairly certain this method won't boot from removable disk (or my FireWire one at least). I get mount_hfs usage line *after* launchd has started and the root filesystem is mounted and then a cleanup/sync and reboot; I'm working on taking a look at rc (having it echo what it's doing - it's calling mount_hfs incorrectly somehow while trying to mount scratch).
Also: anyone who wants to try to get by with a lower-than-.4.8 kernel - don't bother, it won't work.

UPDATE: $SCRATCH is coming out null; redirect --scratch isn't returning anything. I don't know where redirect gets its scratch values from but something's definately not there.
I'm going to use the ghetto solution and point $SCRATCH at RAM.
UPDATE2: Setting SCRATCH="ram://16384" makes the system boot to the WindowServer blue screen of infinite wait. And AppleTV restore image naturally doesn't keep very good logs. *sigh* going to try increasing the scratch size.
UPDATE3: I set SCRATCH to a new HFS+ partition so I didn't have to worry about size and so I could recover logs. It appears (from logs) now blued is messing up some pointers and dying. Repeatedly. Going to remove Bluetooth kexts and see if it keeps blued from starting.
UPDATE4: Moved IOBluetooth* out of Extensions and seems to be working. now I just have to figure out why the BackRow patch I ended up with keeps failing hardware assertations and find a working one.

MacBook + External Drive Summary: Performed steps listed in semthex's above instructions; edited OSBoot/etc/rc to use SCRATCH=/dev/disk1s3 (an HFS+ partition). Deleted IOBluetooth* from /System/Library/Extensions. Patch disk0s4 to a real partition for media in BackRow.framework [at offset E03E0]. This is *critical* and will fix the failed assertions.

Existing issues: Wireless and video playback aren't working for me from AppleTV OS, though the wireless kext is loading. I'm working on it. For now, I'd recommend just using a real osx.

neg: The "kickass intro video" (shiny apple) is in AppleTV boot.efi, which only works on a real AppleTV, so no, sorry

If anyone smarter has the faintest of clues why redirect --scratch is null (if I don't have a disk partition shouldn't it auto-set to the proper RAM value?) ideas would be appreciated.

It did't work for my Macbook!!
Whatever I changed any files like Finder.app,Boot.EFI,etc...
When AppleTV OS loaded for a while,it just stand on black Back and a White Apple,
then nothing happened.
What's wrong with me?

PPC Macs, Intel Macs or Both?