Looks like some of the Delta grounded flights might be true. The article says 20 mainline and 20-25 regional jets.
Delta Grounds Several Bombardier Planes
The United grounding (or selling) planes seems like a possiblity too. CNN says as many as 20 737's.
United plans to shed at least 20 planes - Mar. 18, 2008
Frontier is selling 4 planes.
Frontier Airlines Media Room - News Releases
Most of these articles point to the rising cost of fuel. I heard on NPR a couple of weeks ago a discussion on the rising cost of fuel. And
they said that supply and demand of fuel only accounts for oil up to about $89 a barrel that the increase above $89 is something else. In their discussion, they were talking about the subprime mortgage industry and other shaky financial industries (this was before Bear Stearns) making investors leery of parking their money there. So the investors start looking at other "safer" places to invest, ie oil.
Hear though a grapevine discussion that SWA was the best situated of all the airlines because they purchased huge quantities of fuel futures when the price first began to rise. But that is just grapevine speculation.