Okay, time to stop testing and time for bed, but here's what I've found:
From outlets around the tank (far from the panel) the potential between Neutral & Ground is 0.0 -> 0.1 (sometimes, but it just bounces there for an instant so if you blink you'd think it was always 0.0VAC). But on some other circuits (mainly in the basement, close to the panel) the Neutral/Ground potential is up closer to 0.5VAC.
I found two outlets in the house are wired wrong (hot and neutral are reversed). They each only have very minor loads on them. Shutting down these two bad circuits (and their loads) didn't remove the 0.5VAC potential.
Another 2 outlets in the garage (single garage circuit) don't work at all - but their immediate upstream outlet works, so I'm thinking a loose connection inside either the last working or the first non-working outlet... I will check out the garage properly tomorrow when it's light out.
I have not tested quite all the outlets in the house yet - family sleeping, etc... there very likely may be another outlet with a load on it that is wired backwards feeding current incorrectly -- or heck, possibly feeding return current down the ground.
Never a dull moment
Craig