How can I re-run the initial Setup Assistant on Mountain Lion?
Actually, you should be able to do it by removing a hidden system file which tells OS X that setup has run. Delete this file:
/var/db/.AppleSetupDone
It's a hidden file, so you will have to remove it using Terminal, or show hidden files in Finder. You can remove the file like so:
rm /var/db/.AppleSetupDone
To show hidden files in Finder, type the following into the Terminal:
defaults write com.apple.Finder AppleShowAllFiles -bool true
…and then restart Finder.