I'm sitting here right now in the dead of night and it's very quiet, I've got my canalphones in but no music playing. With my iPod on I hear a very faint hiss because power is going to the headphones. When I turn off the iPod the sound dissapears completedly, no power is going to the built in amp and headphone port. Now when I put the 'Hold Switch' ON, the faint hiss reapears. Not good for saving battery life.

EDIT: Scratch that, the hiss (with the hold switch ON) dissapears after 5 secs