"Steven P. McNicoll" wrote in message
It isn't strictly "first come, first served". Aircraft in distress have
priority over all others, "Lifeguard" aircraft are given priority over all
but aircraft in distress, etc., etc., etc.
http://www.faa.gov/atpubs/ATC/Chp2/atc0201.html#2-1-4

Thank you,
D.