United Airlines and Continental Airlines now charge $150, up from the previous $100, to change a non-refundable ticket on a domestic route. International tickets are now $150-$250, if they can be changed at all (some international fares are "you buy it you fly it"). And some international fares can be changed only before or only after travel. So it's good to check before you buy. See United's FAQ on changes.
As near as we can make out, tickets bought before the increase (April 19 for United and April 24 for Continental) can be changed for the old $100 fee.
American, Delta, Frontier, Midwest, Northwest, and US Air are holding to the $100 fee for now, but we don't expect that to last. If you think you might need to change your flights, stick with Southwest, which does not charge a change fee (however, if the fare has gone up since you bought your ticket, you'll have to pay any fare difference; and if you change on the day of travel, you'll pay the full walk up fare). Or go with one of the newer low cost carriers.
Airtran charges $75 (unless you change within 4 hours of making your reservation); Virgin America charges $40, JetBlue $50, and Spirit $60-$70.
