The parameter for Mac OS X Mountain Lion defers from previous versions, I had a v.1 extension which I had to alter after upgrading to Mountain Lion. The following should clarify things:
# show hidden files pre mountain lion
defaults write com.apple.Finder AppleShowAllFiles true
# hide hidden files pre mountain lion
defaults write com.apple.Finder AppleShowAllFiles false
# show hidden files mountain lion
defaults write com.apple.Finder AppleShowAllFiles 1
# hide hidden files mountain lion
defaults write com.apple.Finder AppleShowAllFiles O