Attack of the Machines - Is your stockbroker a robot? By Daniel Gross:

"But as program trading has risen, the market counterintuitively seems to have grown less volatile. "

Only if you the nature of markets isn't part of your intuitive understanding. It makes perfect sense, if you understand that:
a) markets spontaneously order, volitilaty is reduced by decreased ineficiency
b)behind every computer program is a person and therefore the market
c)computer arbitrage allows people to spot and react to small changes in the market more quickly, reducing inefficiency