1. A system for facilitating diagnosis and maintenance of one or more control networks, comprising: an onboard vehicle control network located on a mobile conveyance; a wireless interface coupled to said control network; a wireless ground station configured to communicate over a wireless communication channel with said onboard vehicle control network via said wireless interface; a local area computer network in communication with said wireless ground station, said local area computer network comprising a server computer; a database comprising diagnostic information relating to said onboard vehicle control network; and a wide area network interface, whereby additional diagnostic information relating to said onboard vehicle control network is obtainable from one or more remote computers; and a portable handheld wireless diagnostic unit with a manual input interface and graphical display, said portable handheld wireless diagnostic unit having no physical connection to said onboard vehicle control network and configured to: communicate wirelessly with said onboard vehicle control network via said wireless interface; convey instructions wirelessly to the onboard vehicle control network in response to input from said manual input interface; and communicate wirelessly with said local area computer network via said wireless ground station, thereby receiving the diagnostic information from the local area computer network via said wireless interface pertaining to the onboard vehicle control network.
8. The system of claim 1 wherein the portable handheld wireless diagnostic unit is configured to send commands originating from the portable handheld wireless diagnostic unit to the onboard vehicle control network, said commands being generated in response to inputs entered via said manual input interface.