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There is a difference between bonding and grounding. The copper piping system inside the house should be BONDED to the service entrance ground. See article 250.104 of the 2002 National Electrical Code. The purpose of this is to provide a path for fault currents should an energized wire short to the piping system someday, somewhere inside your house. The service entrance cannot be effectively GROUNDED to a water system supplied through plastic piping, and there is no requirement to attempt such a thing. There are severe limits on trying to use even metallic underground water piping to accomplish effective grounding. See article 250.52 of the NEC. Some local jurisdictions require that your well casing be grounded to the electrical system ground; typically this is done via the grounding wire to the pump motor. Oftentimes, this ground is a much lower DC resistance than that afforded by the typical driven electrode. Such a situation is not without potential problems, however, as the flow of leakage currents through the well casing is said by some to lead to electrolytic deterioration of the casing, and to water quality problems.
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