In case you haven't heard the news, and I'll bet a lot of people haven't, next year United will begin charging not just miles but also a copay if you wish to upgrade your economy class seat.
Efffective Mar 1, 2007 American began charging $150 plus 15000 miles each way from most domestic travel between the Continental US/Canada/Mexico/the Caribbean to/from Hawaii; and up to $700 plus miles for foreign destinations.
American also began charging, effective Oct 1, 2008, a $50 copay to upgrade most discount economy fares within the continental US and Canada, and within/between the continental US, Canada, Mexico, and the Carribean. Note that these copays are not refundable should you change your mind (plus, of course, you'll have to pay a penalty of up to $150 on most airlines to re-bank your miles).
Effective July 1, 2009 United is adding co-pays to all frequent flyer ticket upgrades. Previously, it was just pay 5000-15000 miles each way from an economy class ticket; now it's miles plus $50 from most fares within the continental US; up to $600 round-trip for foreign destinations.
Continental already charges most frequent flyers up to $1000 RT plus miles to upgrade to international BusinessFirst; and up to $300 RT plus miles for domestic.
The airlines call these "copays" but they're really just new fees, and they make frequent flyer miles even less valuable than before, and, of course, they're in addition to all the other fees that airlines impose on frequent flyer tickets, such as re-banking miles and "close-in" ticketing (as if it really costs them anything to issue an electronic ticket a few days before you fly. Most airlines allow you to do it yourself online with no human intervention. Please).
In the past, we've recommended cash back cards (especially the American Express Blue card) rather than frequent flyer credit cards, except if you use your miles for upgrades or for business and first class tickets. But with these new fees, cash back cards are looking better and better, even for premium fares.
Near as we can make out, US Air and Delta/Northwest haven't added these new fees, but we have no reason to expect that they won't follow suit.
And perhaps most galling, not to beat a dead horse, frequent flyer fees are nonrefunable. Imagine a worst case scenario: you book an award ticket last minute on United ($100 fee) on an international route ($500 fee) and then something comes up and you can't make the trip ($150 fee to redeposit). You're out $750 and you never left the ground.