1. A fuel processing system, comprising: a feed assembly adapted to deliver at least a carbon-containing feedstock to a fuel processor; a fuel processing assembly adapted to receive a feed stream containing at least the carbon-containing feedstock and to produce a product hydrogen stream therefrom; and a fuel cell stack that is adapted to receive an oxygen-containing stream and at least a portion of the product hydrogen stream and to produce an electric current therefrom; and a control system that is adapted to regulate and control the operation of the fuel processing system responsive at least in part to inputs from a sensor assembly that includes at least one sensor adapted to measure an operating parameter of the fuel processing system, wherein the control system is adapted to automatically transition the fuel processing system between selected ones of a plurality of operating states responsive at least in part to inputs from the sensor assembly, and further wherein the plurality of operating states includes a running state in which the product hydrogen stream is produced by the fuel processing assembly and at least a portion of the product hydrogen stream is delivered to the fuel cell stack, and an idle state in which the fuel processing assembly is maintained at an operating temperature and an operating pressure for producing the feed stream, but no more of the product hydrogen stream is delivered to the fuel cell stack than is required to operate the fuel processing system in the idle state.