Should There Be Rules of Combat for Cyber Warfare?

The Pentagon is about to make public its conclusion that computer sabotage from another country can be an act of war, possibly justifying a military response. One official told the Wall Street Journal, "If you shut down our power grid, maybe we will put a missile down one of your smokestacks." But how could the US be sure where the attack came from, how much damage it really did or what level of military response would be "equivalent?" Is the US rattling sabers in cyberspace? Does peace in the virtual world require diplomacy, rather than threats of retaliation?