Do you see it if you log into a test account? If you don't have a test account, go to System Preferences > Accounts and create one. If it's gone from the test account, you may have a corrupt preference file in your regular account. Move ~/Library/Preferences/com.apple.dock.plist to the desktop. Log out/in. If the problem is solved, reset the dock to your specifications and trash that plist. If not, put it back and then log out/in.
