I believe all the supply transformers have the neutral grounded (but I'm not sure of that), but yes, each house has an earth ground that ties to both the house neutral as well as the house ground wires. The intent of the ground wire in the house is to handle any ground fault current (which is why most of the requirements indicate that the ground wire must be at least as large as the current supply wires). When I put my addition on 20 years ago, the electrical inspector verified that all my grounds were connected at the panel bus where the neutral wires also joined. He also checked that none of my neutral and grounds were connected together anywhere other than at the panel. He also had me verify the grounds for the hot tub were solidly connected back to the main ground (I put in a second ground rod and bused the two together (which he liked). The neutral wire that comes into the house from the meter is also connected to the ground bus in the panel. Yes, there is a potential for house A to be slightly different from house B depending on the loads etc. I used to have a friend years ago that was a commercial electrician and we were talking about grounding one time and he commented that "you should see the size of the heavy copper bus bars that run around in hospitals to make sure everything is REALLY at ground potential".