In examining the strings inside the Dock binary in Leopard, I found a way to add real separators to the dock, instead of adding dummy applications with empty or decorative icons, as commonly done.
This works only in Leopard (OSX 10.5):
Run the following commands from the Terminal:
defaults write com.apple.dock persistent-apps -array-add ‘{ “tile-type” = “spacer-tile”; }’
killall [...]