More pressure reduces rolling resistance up to a point; above some sweet spot you start losing energy when the tires get too stiff to provide suspension; road bumps that the tires don't absorb are passed on to the bike and you and the energy lost in vibration is energy which would otherwise be hurtling you forwards. The optimum pressure depends on the width of the tire and the total weight that it is holding up (i.e. both you and the bike, divided unequally over the two wheels).

According to this article the best balance of rolling resistance and suspension is with 15% drop: http://www.bikequarterly.com/images/TireDrop.pdf. It includes a graph showing the optimum pressure for a variety of widths and weights.

My racing bike has 700x23 tires and weighs ~190 pounds with me on it; I run 85# front and 110# rear.

My wet weather/rough road/grocery store bike has 700x37 tires right now and weighs ~200 with me, racks, paniers, etc.; I run 40# front and 55# rear. It is incredibly comfortable and actually rides faster than when I had 28mm tires running at 65/90!

Go too low and you risk pinch flats.