AMERICA HAS TWO ECONOMIES, and one is flourishing at the expense of the other. First, there's the legitimate economy, in which craftsmen are licensed and employers and employees pay taxes. Then there's the fast-growing underground economy, where millions of nannies, construction workers and others are paid off-the-books, their incomes largely untaxed. The best guess as to the size of the output of this shadow economy is about $970 billion, or nearly 9% that of the real economy. It should soon pass $1 trillion. What is largely fueling the underground economy, experts say, is the nation's swelling ranks of low-wage illegal...