I thought I would contribute what I did to install Tiger on the ATV so that it boots without having to have a external drive, and never opening the ATV up.

Requirements:

ATV (duh)
Intel Mac
Some way of installing Tiger for Intel, most likely the restore disks for the Mac
2 travel drives. One needs to be at least 1gb the other can be an old 256mb
Some spare time

1. Install a fresh copy of OS X (I used 10.4.9 with no problems yet) from a intel mac (I used a MacBook from '07 I believe)
2. Set up the install on the intel mac
3. Boot back into the normal install on the mac
4. Start terminal and type:
defaults write com.apple.finder AppleShowAllFiles TRUE
killall Finder
5. Delete the mach_kernel on the external drive
6. Download the kernel, and boot.efi for later from here
7. Put it on the external drive
8. Replace the boot.efi that is found in /Volumes/ExternalDrive/System/Library/CoreServices (replace ExternalDrive with the name of the drive with the fresh copy of OSX on it) with the one downloaded in step 6
9. Start terminal again and type:
sudo bless --folder=/Volumes/ExternalDrive/System/Library/CoreServices --file=/Volumes/ExternalDrive/System/Library/CoreServices/boot.efi --setBoot (replace ExternalDrive with the name of the drive with the fresh copy of OSX on it once more)
10. Browse to /Volumes/ExternalDisk/System/Library/Extensions and delete all kexts that start with NVD, there should be 5 if running tiger
11. Browse to /Volumes/ExternalDisk/System/Library/ and delete Extensions.mkext and Extensions.kextcache if they exist, which they should
12. Open /Volumes/YourExternalDisk/System/Library/Extensions/AppleFileSystemDriver.kext/Contents/Info.plist using either a command line text editor, vi, pico, or something like TextWrangler. DO NOT USE TEXTEDIT!
13. Find: <key>IOPropertyMatch</key>
14. Paste the following in right after the next line, it should say <array>:
<dict>
<key>Content Hint</key>
<string>5265636F-7665-11AA-AA11-00306543ECAC</string>
<key>Leaf</key>
<true/>
</dict>
<dict>
<key>Content Hint</key>
<string>Apple_Recovery</string>
<key>Leaf</key>
<true/>
</dict>
15. Open disk utility
16. Select your external drive
17. Right click on it and click on Information
18. Make note of the "Disk Identifier", it should be something like disk2
IN ALL OF THE COMMANDS BELOW, REPLACE diskX WITH THE "Disk Identifier" THAT YOU FOUND IN THE PREVIOUS STEP!!!
19. Start terminal again and type:
gpt -r show /dev/diskX
(Make note of the START and SIZE that is returned for the second partition)
20. Now type:
disktool -u diskX
21. Then:
gpt remove -i 2 /dev/diskX
22. If the external drive had more than one partition, meaning more that one you could see in the Finder, that run the following, if not, then skip this step:
disktool -u diskX
23. Run the following command replacing START and SIZE with the values that you found in step 19:
gpt add -b START -s SIZE -i 2 -t "5265636F-7665-11AA-AA11-00306543ECAC" /dev/diskX
24. After I did this Finder told me that the volume couldn't be recognized, or something like that. This is good.
25. If you did everything correctly, then you should be able to plug the external drive into your ATVs USB port and turn it on, and it should boot OSX. You can choose to keep it this way, or you can proceed on to the next step to write those contents to the internal HDD.
26. Download: http://mirror.sov.uk.goscomb.net/ubu...sktop-i386.iso
27. Also download the latest version of atvusb-creator, found at this site.
28. Use the atvusb-creator to format your 256mb as a ATV-Bootloader
29. Open Terminal, and run the following command (replacing where the img should go, and also replacing where the iso you downloaded is):
hdiutil convert-format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso)
30. Run the following command, then insert your 1gb flash drive, then run it again and make note the new device:
diskutil list
31. Run (replacing X with the disk number found above):
diskutil unmountDisk /dev/diskX
32. Run (replacing the path with the path you set in step 29, and replacing X with the drive found in step 30):
sudo dd if=/path/to/downloaded.img of=/dev/diskX bs=1m
33. Plug in the 256mb and the 1gb flash drives into the apple tv
34. Start the apple tv
35. If all goes well, you should see the linux penguin sitting on the apple tv. You may have to temporarily unplug the OSX drive for it to boot into linux.
36. Once the screen displays a prompt, enter root for the username, and root for the password
37. Now type:
mkdir tmp
mount /dev/sdc1 tmp
kexec --load tmp/casper/vmlinuz --initrd=tmp/casper/initrd.lz --command-line="boot=casper initrd=/casper/initrd.lz nosplash vesa"
(This should return some information)
kexec -e
38. Now wait for ubuntu to load
39. Once the GUI has loaded, navigate to Applications>Accessories>Terminal
40. Now find the external drive with osx on it, for me it was sdc1, it will probably be different for you.
41. Now type the following replacing INPUT with the drive you found in the last step:
dd if=INPUT of=/dev/sda1
42. Wait. This would be a good time to catch up on your reading, or watch some tv.
43. Once dd has finished (it should have spit out some information), your done. You should be able to unplug all external devices, besides keyboard, and mouse, and the ATV should boot into Mac OS X!!!

Sources:
http://www.mark-up.com/apple-tv/install-osx/
http://www.mythtv.org/wiki/Installin...Mythbuntu_9.10
http://www.ubuntu.com/desktop/get-ubuntu/download

Comment if you have any trouble, or if it works!