Phil Hobbs wrote:

(...)

I'm not an electrician, but I believe that most houses in the US have
two-phase service,

Nup. Single phase, center tapped.

http://en.wikipedia.org/wiki/Split-phase_electric_power

and that the neutral is bonded to the ground rod near
the service entrance. Voltage drops in the neutral make one side go down
in voltage and one side go up.

Yup.

--Winston