Farecast, a sytem that uses hi-tech to give fare chasers more power has been in the works for a while:
Farecast, formerly known as Hamlet Inc., will utilize predictive technologies and data-mining techniques to deliver an innovative, leisure-focused, online travel experience. The company has evolved from a University of Washington research project, led by computer science professor and Internet search expert Oren Etzioni, into a venture-backed company. Farecast is located in Seattle, Washington, and is funded by Greylock Partners, Madrona Venture Group and WRF Capital.
TechCrunch just posted about FlySpy - a company that looks like it might make it to market sooner than Farecast. The TechCrunch overview contains an image of a timeseries showing dates and ticket price changes. One thing to keep in mind, this is not like predicting weather: a natural, but complex, system. There are folks on the other side of the price changes that are also running their datamining, predictions and optimization algorithms. It's going to be more like an arms race.