The attached patch shows the usage information when no argument is given to apt-mark.
--- apt-mark.orig 2010-03-04 16:23:23.450331213 +0100 +++ apt-mark 2010-03-04 16:27:30.974347744 +0100 @@ -82,6 +82,9 @@ else: STATE_FILE=options.filename + if len(args) == 0: + parser.error("first argument must be 'markauto', 'unmarkauto' or 'showauto'") + if args[0] == "showauto": show_automatic(STATE_FILE) else: