I generally agree with Angelo's answer. Tire diameter and your weight will determine the optimal pressure. Beyond that, this is actually a controversial subject.
Let's assume that you want to minimize rolling resistance. "Rock hard" isn't necessarily the best approach. One widely cited opinion is that above a certain pressure, there's no benefit in terms of rolling resistance, and the target is a 15% "drop" (vertical deformation under load)—it is argued that even with less drop than that (i.e., higher pressure) you do not see an improvement in rolling resistance, so all you're doing with that extra pressure is making the ride harsher.
Here are a couple of articles (pdf) discussing pressure, rolling resistance, and tire drop. You can see that there's a diminishing-returns curve in terms of power savings at higher pressures. The second one includes a graph showing how much you should inflate a tire to get a 15% drop depending on how much load there is on that wheel and how wide the tire is.
I should point out that this opinion is not universally held. Jobst Brandt, for one, is skeptical, but I'm not sure what he recommends.