I have a friend who sent me a bunch or articles showing that lower psi is better than high psi. There is no measurable increase in rolling resistance but your ride is smoother and more comfortable. In fact lower pressure is better for when you hit bumps, because your tires roll over them more easily so your momentum keeps going forward instead of being interrupted by the bump. If I can find them I'll post the articles here, but I have to warn everyone that they will make your eyes glaze over -- it's very dry reading.

Anyway, I used to keep my tires at 120 psi. I agreed to experiment with lower pressure and I'm averaging the same speed at 110 psi. One of these days I will drop it to 100 psi and I won't be surprised if it makes no difference in speed. I also mentioned the experiment to the mechanic at the LBS and he said he wouldn't go above 100.