This is just some data on a potential (ephasis on the word "potential") problem associated with having multiple paths to ground, particularly with audio and video systems:http://www.epanorama.net/documents/groundloop/basics.html
"A ground loop occurs when there is more than one ground connection path between two pieces of equipment. The duplicate ground paths form the equivalent of a loop antenna which very efficiently picks up interference currents. Lead resistance transforms these currents into voltage fluctuations. As a consequence of ground-loop induced voltages, the ground reference in the system is no longer a stable potential, so signals ride on the noise. The noise becomes part of the program signal.
Ground loop does not cause problems when all of the following thing are true:
1. None of the wires in the loop carry any current
2. The loop is not exposed to external changing magnetic fields
3. There is no radio frequency interference nearby
If there is any current folowing in any wires, there is then some potentital difference which causes current to flow in other wires also which causes problems. The loop will also act as coil and pick current from the changing magnetic fields around it. Wire loop acts also like an antenna picking up radio signals."
Don't flame me I didn't write it. Don't ask me to explain it I only loosely understand it my self. Where did I find it? Well somewhere there is a really hot librarian chick in the back book storage room with her hair falling out of its bun and her classes on sideways from bouncing up and down while straddling the maintenance guy on the floor and screaming GOOGLE!