There is no such thing as absolute voltage. There is no such thing as just '0V'.
At the risk of adding to the confusion, there actually IS such a thing as absolute voltage. This was one of the earliest electrical discoveries, when electricity was more of a scientific curiosity than a useful way of powering things.
An electroscope measures absolute voltage.
http://www.school-for-champions.com/experiments/static_electricity_electroscope.htmhttp://en.wikipedia.org/wiki/ElectroscopeThe foil plates of the electroscope repel each other when they're positively charged, and they also repel each other when negatively charged. They don't repel each other when they're at zero volts.
To be fair, the typical amateur-built electroscope is a very crude instrument, and only shows easily measurable repulsion at relatively high (positive or negative) voltages.
The idea that voltages are always relative, never absolute, is a very useful practical model for the level of voltages we typically encounter in most electronic circuits, where current flows. And it's absolutely true that an ordinary voltmeter always has two terminals, and measures a potential difference between those two terminals. But that "no absolute voltage" model doesn't explain how an electroscope's foils can repel each other at positive and negative voltage, with no repulsion at zero volts.
---ok, feel free to ignore the above if you must---
Trying to bring it back to smoothtalker's misconception, the "hot" wire swings between about +170V and -170V with respect to neutral. When the hot wire is higher voltage than neutral, electricity flows one way. When it's lower voltage than neutral, electricity flows the other way. Power is dissipated regardless of which direction the electricity flows, so the filament heats up both ways.