MileValue has compiled a helpful chart of the (often substantial) fees that airlines charge for last-minute bookings using frequent flyer miles and points.
Some airlines, like Air Canada, Alaska Air, British Air and KLM, do not charge a fee.
Others, like American and United, charge $75. Still others, like Delta, JetBlue and Southwest, charge a hidden fee.
The blog post links to a method of avoiding the $75 fee on United.