Justin posted these commands on the old forum (copy/paste into Terminal):
Hide icon
gsettings set net.launchpad.plank.dock.settings:/net/launchpad/plank/docks/dock1/ show-dock-item false
Show icon
gsettings set net.launchpad.plank.dock.settings:/net/launchpad/plank/docks/dock1/ show-dock-item true
Holding CTRL down and rightclick on any plank icon, will let you go into preferences.