Displex works.
I have a new black Nano, with a major dulling and matt area on the black front and screen caused by Nail varnish remover (don't ask!) a large patch that is much worse than scratching. I bought Displex (£4on UK ebay).
I applied it with a spectacles cleaning cloth and after 30 minutes of careful circular polishing, medium pressure, it has made an improvement of say 90%. Casual lookers will not notice the original damage. Another session of 10 minutes will probably cure all trace of damage, although wife is now happy that her precious Nano is not terminally scarred as it was.
G5 D2.3GHz, Powerbook 15inch 800, G4 D1GHz, imac DV 500MHz Mac OS X (10.4)