With the quantity of fuel airlines burn in a year, they usualy buy their fuel and pay for it sometimes years in advance. A friend of mine is a regional chief pilot for SouthWest airlines in Baltimore and he was telling me the only reason they posted a profit last quarter was the fact they had paid for their fuel back in 2003 at 2003 prices. He also mentionned that 85% of their fuel burn for 2005 was purchased and paid in the first half of 2004, so looks like they will be able to keep their ticket prices somewhat below their competition.
As for AC, the way they manage their investor's $$ wouldn't surprise me if they were on a pay as you burn plan. My 2 cents.
Corporate usually pay the posted price at the FBO. If you go to a specific FBO on a regular basis, you can have "contract fuel" which is basically a yearly agreement of fuel purchased and is directly billed to the operator bypassing all credit cards fees, airmiles and frequent flyers points thus allowing the vendor a little more room to give you a little break on the price.
hope this helps,
Dan