From the New York Post today:
A new fee of $25 for a second bag is being levied by Continental, Delta, Northwest, United and US Airways. And low-fare carrier AirTran announced that it will charge $10for a second checked bag.
Airlines have long levied fees for checking a third bag. Some carriers have recently increased those as well, and the fees on the major carriers now run as high as $100.
As for the extra-bag fee, even those who say they pack lightly for their trips foresee problems caused by price-sensitive fliers overfilling their carry-on bags and using large amounts of scarce overhead-bin space.
Here is a great site about traveling light: www.onebag.com