How risky are stocks?

The widely held view that stocks are not risky in the long run is attacked by a nice article in today's WSJ.

It states:

Look at the long-term average annual rate of return on stocks since 1926, when good data begin. From the market peak in 2007 to its trough this March, that long-term annual return fell only a smidgen, from 10.4% to 9.3%. But if you had $1 million in U.S. stocks on Sept. 30, 2007, you had only $498,300 left by March 1, 2009. If losing more than 50% of your money in a year-and-a-half isn't risk, what is?