I can't figure out why you guys are so desperate to 'deface' something as lovely as the iPod!
Finger marks and smudges? Get a cotton duster, breathe on the back of the unit and wipe. Simple.
Scratches? Be more careful about placing the iPod in your pocket together with loose change/keys etc. Or buy a case.
Mine is over 2 years old. Is it scratched? Hell yes! But the occasional going over with iClean fixes that. It's not perfect but I'm not trying to kid myself by trying to make the unit look like 'new'. It gets used. If it was a cheap cassette player or a MD player then I would expect similar levels of wear and tear.
Also, I can't help feeling that unless you're exceedingly good, you're going to end up with an 'amateur' job - much like if you get a can of spray paint at the auto shop to do a panel on your car, it's not going to look professional.
As for warranty... I couldn't be 100% sure that such a mod doesn't invalidate it.
Just chill and be content with the best player on the market.