United is in T3 and US Air is in T1. They are walking distance from each other - via T2 (the empty terminal), but plan on a 15 minute walk as United is at the far Left of T3 and US is the very last airline in T1.
I believe that you can use the AirTrain / AirTran between the terminals.
FYI - I walked it the other day and it was not horrible - but my shoulder bags were getting heavy by the time I got to T3.