Yesterday, Kottke requested a service that would provide guidance on when to purchase a plane ticket in order to get the cheapest price and still snag a ticket before a flight sold out. It appears that Farecast may provide such an online travel service when it launches.
I'd heard of a technology called Hamlet a couple years ago. Developed by a UW Comp. Sci. professor, the computer program used past airline price trends to predict future prices. Tests showed the program could save passengers money in the long run.
Since most airlines can't turn a profit, I'd imagine that they shift prices with the expectation that some tickets will be sold at higher prices, other at lower prices. If customers start using Farecast en masse, the airlines will have to respond by altering their algorithms.
Some things won't ever change, even with tools like Farecast. Your best bet for minimizing your airfare is still to start shopping way in advance and to be willing to fly at odd times.