Short-term debt issued by Exxon Mobil and Johnson & Johnson is yielding less than U.S. Treasuries of comparable maturities, upending the usual situation in the bond market, the Wall Street Journal reports. An Exxon Mobil issue that matures in 13 months is yielding one basis point less than the comparable Treasury, as is a Johnson & Johnson issue that matures in 2014.

Of course, both Exxon Mobil and Johnson & Johnson have AAA credit ratings, among the few corporates that still do, while Standard & Poor's cut the U.S. government's rating to AA-plus last year.

The Journal notes the flood of money that has moved into corporate bonds recently as investors hunt for yield, as well as the shrinking supply of top-rated corporates. And given the looming fiscal cliff, investors fear the U.S. credit rating could be clipped again.