We've been advocating the policy that some airlines follow to refund the fare difference if a fare goes down in price from the time you buy it to the time you fly. You have to keep your same flight or at least the same travel dates and class of service to qualify for a refund, but some airlines will refund the entire difference without a fee, and others charge a fee of up to $100 on a domestic fare. US Airways has flip flopped over the years on this, charging the fee one month, and not charging it the next.
In July, US Air flipped again, and now they charge $100, often wiping out any savings. So currently, the only major airlines that will give you a voucher for the entire difference are United, Southwest, Alaska, and JetBlue.