Well I got my orange 2G Shuffle without reading any reviews, because I decided that it's what I wanted for exercise and easy syncing with my pre-existing iTunes library. Until just now I'd only been using it for that task, and no critical listening, but I thought I'd come here to check out what people thought of it.
This "hiss" problem seems to be pretty damn small on this model. I have to put the volume so low as to hardly be able to hear the song to notice it at all (using marshmallows). Using Grado SR80s it is completely inaudible - they are hardly high impedance!
If a song isn't playing, then using the marshmallows you can make out the slightest hiss. I checked my colour iPod 4G as a comparison, and the paused-mode hiss is indeed louder on the Shuffle. However, at a guess I'd say it's at (the very) least 10 times more quiet than general recording hiss.
Perhaps I just dropped lucky?
Either way when exercising I'll never hear it, ever.