I've done some digging around, and here is what I found from their Contract of Carriage (COC). It does not state that they will put you on another airline, except maybe if the flight is diverted. What people need to understand is there is no law that says they have to put you on another airline. This is why Southwest does not do this normally, although in some extreme cases, they will buy a ticket on another airline for a passenger.
http://www.aa.com/aa/i18nForward.do?...age.jsp#Delays
When cancellations and major delays are experienced, you will be rerouted on our next flight with available seats. If the delay or cancellation was caused by events within our control and we do not get you to your final destination on the expected arrival day, we will provide reasonable overnight accommodations, subject to availability.
In extreme circumstances, it is possible that a flight will cancel while on the ground in the city to which it was diverted. When this happens you will be rerouted on the next American Airlines or American Eagle flight with available seats, or in some circumstances on another airline or some other alternative means of transportation.