In Safari 6 the behavior of the delete key has been changed. If want to return the ‘page-back’ navigation behavior to the delete key, you can do so with the following defaults write command.
defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2BackspaceKeyNavigationEnabled -bool true
Restart Safari for changes to take effect
If you want to revert to the default Safari 6 behavior:
defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2BackspaceKeyNavigationEnabled -bool false