advise your electrician of your upcoming pipe changes so he connects to
the metal cold pipe that supplies water near its entrance to your home.
have him check your wiring to see if any previous homeowner handymen
have used the water line for electrical grounding anywhere else inside
your building, perhaps at the older washing machine for example.
see Electrical Wiring FAQ for grounding info and more at:
http://www.faqs.org/faqs/electrical-wiring/part1/

Eigenvector wrote:
I'm not second-guessing my electrician, just gathering information for
later.

We agreed to install 2 ground rods, per code, and then tie a second ground
to the water supply line, also code I am told. With that they are also
going to connect a ground between the hot and cold pipes at the water
heater.

My question, what does the ground at the water supply do that the 2 ground
rods don't?
second question, as I intend on replacing all of my hot water lines with
PEX, obviously the ground at the water heater will no longer exist - am I
creating a dangerous condition or can I safely remove the metal hot water
pipes and replace with PEX - so that I no longer have any metalic hot water
pipes?