When Nouriel Roubini talks, the world listens. Roubini is, of course, the once-obscure New York University economist whose dire warnings about a financial crisis proved depressingly prophetic. Last week, Roubini was shouting. Writing in the Financial Times, he warned that the Federal Reserve and other government central banks are fueling a massive new asset "bubble" that -- while not in imminent danger of bursting -- will someday do so with calamitous consequences.