*** this is a repost... i wrote this up before when i was showing how to trash a mac ***
*Note: This methos will not work if the harddrive is encrypted as you will be prompted for a username/password to decrypt the root drive.
Step 1- Shutdown the mac
Step 2- Hold down the "COMMAND" and "S" keys together and press the power button. Continue to hold down the "COMMAND" and "S" keys until you are at a root prompt
Step 3- At the prompt type "mount -uw /"
Step 4- You are now root, you can do whatever you want from here.
Note: If they have put a password on the open firmware you will not be able to do this (like in the Apple Store) but 99.9999% of users do not put passwords on their bootloaders/firmware.