Since the 3G Nanos just came out, there aren't many options available yet as far as cases and covers are concerned. This will change quickly, I'm sure, particularly with the return of the regrettable back mirror-finish. So, list your finds here.
So far, the only ones I've seen are from
SwitchEasy, with the seemingly sleek
Black Biscuit and the equally appetizingly named, multi-color
Color Biscuits.
I'm not sure which one I like best, the white color one, or the black "hidden screen" one, though I'd like to see how the former looks with a silver, instead of a black Nano. That said, they all look nice, but I couldn't find any information on the website as to how much thickness it adds to the Nano, which is pretty basic info.
I'm hoping the other usual suspects (iSkin, Speck, etc.) will have new 3G Nano cases soon.