To Wharvey:

I've seen several posts to your question and I wanted to make sure you knew that airlines MUST give you a credit when your fare decreases (it has to be the same flight, though) and they cannot charge a $75 change fee.

Airline policies differ, however, in how they give you the credit and how you use the credit.

For example, Delta requires you to go to a ticket office or airport to pick up or use a voucher. If a Northwest price drops, however, you can just call and they'll mail the voucher. And when you want to redeem a NW voucher, you can make a phone reservation and mail the voucher - no need to go to a ticket office.