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i like most of the slim cases on Apple's website (you might have to refine the search by selecting "iPod nano (aluminum) from the drop-down menu). link to UK store - the selection is greater in the US.

Radtech also makes a silicone case, called Gelz, and a sleeve, called Sleevz (both links to US store; i'm not sure if Radtech has a UK store).

from what i've seen, the aluminum nanos are pretty resilient so i would guess even an iPod sock would work.
 
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