There are no words in the NEC or MRC that direct us to bond the the cold water, hot water and gas pipe together at the water heater.
The message in the NEC at 250.104(B) fills us in and tells us that the piping system that might become energized is to be bonded using the sizing based on the overcurrent device of the circuit that possibly will be energizing it. This puts us back to 250.122, NEC 2011 Ed. or the MRC 2009 Ed. E3609.7 and E3609.7.1.
Usually this happens automatically with the branch circuit grounding conductor.
Think gas stove, furnace, dishwasher, electric water heater.
Now CSST is handled differently but that's not part of this question.
Having said all that it becomes the Inspector who might make a call that a piping system is likely to become energized. If he does, I'd be interested in how this might happen.
