In their constant search for revenue without raising airfares, airlines are coming up with ever more fees, so United Airlines on July 1, 2009 will require rather hefty "co payments" (i.e., fees) to use your hard earned frequent flyer miles for upgrades (and you thought those rides would be free. Ha).

 UPDATE:

Looks like United has changed the effective date to Jan 12, 2010:

You'll pay:

• $50 for domestic (48 US) upgrades from economy other than fulll fare Y, C, D or B class fares
• $250 to $500 for upgrades to Hawaii from/to the Mainland and on international flights, except Y, C, D or B fares.
• Upgrade awards requested prior to the effective date for travel after the effective date will not be affected, so if you're planning to upgrade, do it before July 1.
  

If there's a silver lining in all this, beginning July 1, 2009, all paid international Economy-class tickets can be upgraded with miles and a co-pay. Currently, only select Economy classes can be upgraded internationally.

But keep in mind that American and Continental charge even more for upgrading. And Delta doesn't charge a copay because on most routes you can only upgrade to business or first class from full fare or nearly full fare. At least United lets you upgrade from a wide range of discounted, non-refundable economy class fares.