Baggage rules have changed a lot since September 11, 2001, and United Airlines is no different in their changes. Rules and fees for overweight bags change frequently, so check before you fly. These rules are as of February, 2009.
Baggage fees in the US and Canada
If you are traveling within the U.S. and Canada, there is a charge for checked bags: $15 for the first bag and $25 for the second bag. You will be charged each way, and the bags should be under 50 lbs and under 62 linear inches (length + width + height) to avoid additional charges.
International travel fees for baggage
Fees vary depending on your Mileage Plus status and/or the class of service that you're flying. If you are flying to or from the U.S. as an economy passenger, excluding Brazil, you can check two 50 pound bags that are 62 linear inches for free.
Traveling to/from Brazil
The weight limit for all travelers to and from Brazil is 70 pounds per bag. All travelers must check bags that are 62 linear inches or smaller. Economy, first and business class passengers can check up to two bags. Global Services, 1K, Premier Executive and Star Alliance Gold passengers can check up to three bags.
Traveling to Mexico City
Economy-class flyers can bring two checked bags, with a 50-pound weight limit per bag, to Mexico City. First and business class passengers can also check two bags, but there is a 70 pound limit per bag.
Active Military Personnel Traveling to Asia
Active military personnel and their dependents with ID who are traveling on orders to Asia can bring as many duffel bags, sea bags or B-4 bags as needed, up to a total maximum weight of 140 lbs.
Active Military Personnel Traveling Internationally Excluding Asia
Military personnel and their properly identified dependents who are traveling on orders elsewhere may check up to three duffel bags, sea bags or B-4 bags that weigh up to 70 pounds maximum per bag, and are, at most, 115 linear inches.