High oil prices

The price of jet fuel has almost doubled in the past year, and together with the slowing economy, is putting the squeeze on airlines. (Southwest, in a prescient move, hedged its fuel price a few years back with long term contracts.)

United is responding, in part, by cutting flights and retiring its older, fuel-inefficient 737s and 747s. And that's how it's supposed to work: higher prices lead to less consumption and/or higher efficiency.

Painful... but necessary.