When a cabin is oversold, the flight manager (in hubs) or duty manager (in out stations) looks at a so called grading list to determine whom to upgrade (top of the list) or off load (bottom of the list) from the flight. This list is the general guideline for the gate agent, if they deviate from it they have to give a valid reason. So it doesn't hurt to ask (or give the famous UA 'speech':
http://www.flyertalk.com/forum/unite...ne-thread.html ) but don't get you hopes up.