IMO this code isnt enforced often..... Technically if the water and gas service are plastic underground, and the cold hot and gas inside are metal you need to bond them all to an electrode or main panel.
250.104(B) Other Metal Piping. Where installed in or attached to a building or structure, metal piping system(s), including gas piping, that may become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 250.122 using the rating of the circuit that may energize the piping system(s). The equipment grounding conductor for the circuit that may energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.
FPN:Bonding all piping and metal air ducts within the premises will provide additional safety.
Section 250.104(B) was revised for the 2002 Code to state that gas piping is treated exactly the same as all "other metal piping" systems within a building.
It's that "may"
thats debatable. And if you can get away with doing it at the water heater - hey why not.
[This message has been edited by e57 (edited 09-22-2006).]