If regulators wait to act until they can say with certainty that a credit bubble is about to burst, they've waited too long.That's particularly true when it comes to the opaque and unregulated "shadow" banking system on Wall Street that has now supplanted regulated banks as the leading source of credit for businesses and

consumers. This shadow system gets its money from big investors rather than depositors, and it revolves around hedge funds, investment banks and private equity funds rather than banks. These shadow banks have made borrowed money cheaper and easier to get, but they have also made the financial system and the U.S. economy more susceptible to booms and busts. And with another giant credit bubble ready to burst - this one having to do with business borrowing - we're about to learn that painful lesson again.