I finally found some research-based evidence about the impact of tyre pressure on fuel consumption, and I was surprised by the results. According to Edmunds.com (who manage CNN.com's automotive websites) tyre pressure has only a modest impact on fuel consumption.
...we saw a modest difference in two of the cars. It might have been more dramatic with different tires on different cars. Experts swear by it; we couldn't really document it. And we wound up wondering if tire technology, like the design in other areas of the car, had improved.
However they did find that large savings could be made by driving less aggresively, driving slower and turning the engine off if stationary for over a minute. And some good news, they found that using A/C or driving with windows open had no effect on fuel consumption. Hurray! I can use my car A/C with a clear conscience!