Snow Leopard... upgrade
Step one - External Drive
In order to complete this upgrade, you need an external drive. As with the Snow Leopard initial installation I used a external Leopard installation on a USB External. I have also backed up my Snow Leopard 10.6.2 installation to a spare IDE drive, connected through a IDE to USB adapter. Either way you need an external bootable OSX installation, Leopard or Snow Leopard, it doesn't matter.
Step two - root
You also need to have a root account enabled on your Snow Leopard installation. To enable it, go to System Preferences > Login Options > (On "Network Account Server" select Join) > Open Directory Utility... > (In the menu bar select) Edit > Enable Root user. Remember this password because you will need it later. If you forget, or you've missed this step (for whatever reason) you can follow this step: Boot your Leopard DVD and on the Utilities menu, select "Reset Password..." Select Macintosh from the drop down and then select root in the user drop down list. Enter a password, reboot. Try to login as root using the password, and you'll be prompted to enter another password (you can enter the same as you did in Reset Password).
Step three - Installation
Download the 10.6.6i ISO and save this to your destination drive (Macintosh) and mount it. When you've mounted go to "/Volumes/Hazard_10.6.6i/System/Installation/Packages/" and run OSInstall.mpkg. Once the installation is complete restart your computer and login to your external drive. Once you've logged in to your External drive, copy the extensions for 10.6.6 to "/Volumes/Macintosh/System/Library/Extensions/" - and delete AppleHDA.kext that is also inside the Extensions folder on Macintosh.
Step four - com.apple.Boot.plist
View the com.apple.Boot.plist text file I have uploaded and enter these values into your current com.apple.Boot.plist overwriting what you currently have. Use the following commands to do so. (Note: Yes, enter all those Kernel Flags in their own keys.)
sudo -s nano /Volumes/Macintosh/Extra/com.apple.Boot.plist
Step five - sudoers
If you were to login to your Macintosh drive now, you will have no permissions. The upgrade has overwritten your sudoers file, so you lose permissions and you aren't enabled as a admin, only a standard user. Follow these steps carefully. You have to edit this file on your External drive. Look for the following, and the part that is listed in red, you are to replace this in the file - replace username with your account username you selected when installing Snow Leopard originally.
# User Privileges specification root ALL=(ALL) ALL %admin ALL=(ALL) ALL username ALL=(ALL) ALL
Save the file and exit Terminal.
Step 6 - Pre-completion
Reboot your computer and on the prompt screen, press Tab and enter "-f -v" (without quotes) into the boot flags for Macintosh. This will load the extensions, because they are not correctly installed yet. We'll do this once we login. When you have booted up successfully (if your Keyboard/Mouse are not functioning, reboot and use the boot flags again). You want to login to your root account.
When you've logged into your root account, go "Apple logo > System Preferences > Accounts > (You) > [Tick] Allow user to administer this computer." - then reboot your computer once more. Use the boot flags to gain access to your desktop. This time login into your normal user account. You should have sudo privileges, so we can install the extensions correctly.
Step 7 - Repair Permissions
Open Terminal and enter the following:
sudo -s chown -R root:wheel /System/Library/Extensions/ chmod -R 755 /System/Library/Extensions/ touch /System/Library/Extensions/ diskutil repairPermissions /
When that completes, again reboot. This time boot as normal. Select your Macintosh partition and you should boot without a hitch. When you've reached your desktop follow the steps listed in the Snow Leopard Installation guide for Keyboard, Mouse and Networking (Ethernet functionality) and you're done.
But, there is a difference with Ethernet. Copy and paste IONetworkingFamily.kext over to your desktop. Show Package Contents until you get to AppleYukon.kext, Show Package Contents and replace Info.plist. Copy and paste the IONetworkingFamily.kext back to your Extensions folder - but repair your Extensions using the Terminal codes above - not by using Kext Utility.