NEW YORK (Fortune) -- Here we go again.

Less than two years after the housing market collapsed, the U.S. economy is threatened by a new bubble in asset prices. This time, four billowing balloons are hovering: two commodities -- gold and oil -- stocks, and government bonds.

Don't be fooled into thinking that last week's 5% drop in the S&P, and the recent sell-off in oil, remotely makes them fairly valued, let alone bargains. Equities and commodities, as well as Treasuries, which actually rallied as stocks dropped, still have a long way to fall. The reason: They've already seen huge run-ups that put their prices far above their historic averages, and far above the levels justified by fundamentals.

Two examples: Most companies can't possibly grow earnings fast...