Here is a nice illustration of what happens inside one's electrical panel:



The important details are that the either phase of the hot side could be "seeing ground" across the neutral connection as the neutral from the electrical company meter / service entrance is attached to ground inside the panel.

The folks at Jensen Transformer (who make an OUTSTANDING range of products to electrically filter / isolate A/C power...) have a very thorough pdf that explains all the gory details on how hum gets into systems -- http://www.jensen-transformers.com/an/generic%20seminar.pdf

A simpler overview is also online-- http://www.epanorama.net/documents/groundloop/home_solving.html

These amateur radio guys also have a nice clear explain how to isolate the power if you want to go that route -- http://amfone.net/Amforum/index.php?topic=13605.0;wap2