Not so long ago, Federal Reserve officials were confident they knew what to do when they saw bubbles building in prices of stocks, houses or other assets: Nothing.

Now, as Fed Chairman Ben Bernanke faces a confirmation hearing Thursday on a second four-year term, he and others at the central bank are rethinking the hands-off approach they've followed over the past decade. On the heels of a burst housing-and-credit bubble, Mr. Bernanke now calls financial booms "perhaps the most difficult problem for monetary policy this decade."

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