Chumba, this did happen to me once on Continental, but it is actually rare. While you usually do see upgrading done at the gate, quite extensively, almost always this is done based on 'payment'. Which can be e500s, regional upgrades, miles. You will never, as a general member, be upgraded on a long flight, even if you offer money (unless you actually offer to buy up to a first class seat and become a regular paying passenger) over a United 1k or GS who has e500s, say.
Now, it's been absolutely ages since I've flown US Airways, they might work a bit differently (all airlines in the US have slightly different rules); and when it happened to me on CO I was on a short flight. UA does it totally by status, and computer drives this over the gate agent until the check-in window for a flight is closed (by which time boarding will have actually started on a larger aircraft).
The way you might get extremely lucky on United is if (a) someone checks in at the very last minute (You can do this airside on internal flights, coming off a connection) (b) this puts econ oversold +1, say (c) gate agent happens to have an empty seat in first and (d) all GS, 1k and 1p have already boarded (since they get to do so first). The gate agent, for the sake of having the plane leave on time (more important than upgrading people per entitlement once boarding has begun) might grab anyone who is a member of the FF program and still in the lounge, rather than delay boarding, fight their way in and grab a 1k member who is in an econ seat and move them up. Even then, on a transcon I have seen them come in and move people up. On a short flight, for the sake of free booze and a pack of peanuts, they won't.
