USAIRWAYS will send you a credit voucher as well if the the price drops on a ticket you've already purchased. I've done it several times. Along with the voucher they send you a nice letter stating that they do indeed have a low price guarantee......if the price drops after you buy a ticket, they'll give you a credit voucher for the difference.
As a-matter-of-fact........I plan on calling them tomorrow to ask for a voucher since the price has dropped on my scheduled January trip which I have already paid for. The only problem is, that TACA has since come out with a new price that is even much lower than the new USAIRWAYS price.
Zebra