SAN FRANCISCO (Reuters) - Dire warnings from Washington about a "cyber Pearl Harbor" envision a single surprise strike from a formidable enemy that could destroy power plants nationwide, disable the financial system or cripple the U.S. government. But those on the front lines say it isn't all about protecting U.S. government and corporate networks from a single sudden attack. They report fending off many intrusions at once from perhaps dozens of countries, plus well-funded electronic guerrillas and skilled criminals. Security officers and their consultants say they are overwhelmed. ...