I use the sock with my black nano. It is cotton, and after a week I see miniscule scratches on my nano,, but it's barely noticeable. I've been waiting for my invisible shield, and I think that'll come in today.
Ironically I couldn't wait til the shield came in, but last night I was reading up and got lots of mixed reviews for the invisible shield. Now I'm a little hesistant to use it and might just keep using the sock until I find a nicer case for it.
But the sock does the job. I use another one for my 20Gig, and I might end up using one for my cellphone when that comes back from motorola.