As the attached graph illustrates, the S&P 500 has recently hit a new all-time high in Japanese yen terms (surpassing its July 1999 peak), while the index remains more than a 1/3 below its September 2000 high in euro terms.
click for larger graph
chart courtesy of Mike Panzner
Aside from any other reasons, perhaps it is Japanese investors who have been lending a helping hand to U.S. equity prices?
Perhaps the relative growth and inflation rates help to explain the difference
Chart courtesy WSJ
See also this interesting comparo from the online WSJ about Europe, Japan, and the US.