If you haven't received your money yet, make sure you do. The airlines are greedy sneaky little fucks.
You may not have known it, but during last month's federal budget debate in Washington, the Federal Aviation Administration (FAA) shut down, and with it went sits ability to collect taxes from airlines on passenger ticket sales. That didn't stop airlines from charging those fees and pocketing the money though. If you flew during that time, you're due a refund.
While the FAA was shut down, its legal authority and its ability to collect taxes from the airlines expired. Airlines didn't lower their ticket prices however, and instead raised their base rates to pocket the extra $20 to $40 per ticket. Partially due to pressure from Congress and the IRS, Delta and US Airways have agreed to refund customers the difference.
Ideally, other carriers will follow suit, and all of them will automatically refund you the difference to the credit or debit account used to buy the ticket. If they don't, you can contact the airline and ask, or the Internal Revenue Service (IRS) says you can submit a claim to the them (although they note they're still investigating whether to pay the claims or direct airlines to do so.)
The IRS has published an FAQ on the topic so you can see whether you're eligible for the refund and how much you're entitled to based on the tax rate applied to airlines.