This is all dependent on what the reason is for the delay. Weather and ATC delays do not count. If it's mechanical or something that is "within" the control of the airline, then they are obligated to adhere to their own terms.

Carry a copy of their published terms of transportation with you and demand compensation in accordance with their own rules when things go south.

http://www.usairways.com/common/reso...f_carriage.pdf