No you do not need a key to start any of the commercial airliners I have flown. You do need quite a bit of knowledge in order to get the start procedures done -- even the minimum required to accidentally start an engine.
Sorry Iain, most "valuables" are removed from AA overnight aircraft prior to its overnight stay. If not removed it is probably because it (the "valuable") will not work without being connected to the aircraft (i.e. the DVDs).