<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Steve M:
to respond to Eugene's question: On most airlines, if you're not physically present at the boarding gate 20 minutes prior to scheduled departure time, your seat assignments can be released. This is true even if you've checked in before that time elsewhere at the airport. The fact that you made it there prior to "final boarding" is irrelevant if this occurs less than 20 minutes prior to departure. </font>
Not sure about "most airlines", but on AA (and AFAIK, DL, UA, etc.) if you've checked in and have a valid boarding pass with a seat assignment, it is not to be given away provided you board the plane during regular boarding time (on AA, you must be onboard the aircraft and in your assigned seat at least 10 minutes before scheduled departure time). There is no requirement to check in (or even identify yourself to a gate agent) again at the gate.