For years, America's stock markets were the envy of the world, the model for modern trading fast, stable, efficient and for the most part transparent. But after the Dow Jones industrial average plunged nearly 700 points on May 6 before sharply rebounding, that perception changed, possibly for good. The dollar fell to a fresh 15-year-low against the yen in Tokyo on Thursday amid growing speculation that the US Federal Reserve will ease monetary policy next month.