The real answer is in hedging:

From the article:

"Only Southwest has hedges on more than half its fuel this year, according to a report earlier this month from the ATA. Hedges have positioned Southwest to fly using much cheaper fuel than its rivals."

From a recent Southwest Stock research report:

".. the first quarter, Southwest recorded a $133 million benefit from its hedging positions, but the average cost per gallon increased 62.8% to $1.46 per gallon. Still, Southwest fared better than some other airlines that paid more than $2 per gallon, and the company should be well insulated for some time. For the second quarter, Southwest is 75% hedged at $36 per barrel, compared to current market prices of $75. The company estimates its average fuel cost will be in the $1.45-$1.50 range for the second quarter. For the remainder of 2006, Southwest is over 70% hedged at $36 per barrel; over 60% hedged in 2007 at $39 per barrel; over 35% hedged in 2008 at $38 per barrel; and about 30% hedged in 2009 at $39 per barrel..."

Until the other majors figure out how to hedge their fuel, they will be flying at a competative disadvantage.