My favorite casing solution for mobile devices is offered by WaterField
here. They're dirt cheap (usually less than $15 shipped), extra soft so your device doesn't get scratched, they don't get stuck in your pocket, and they add almost no bulk. If you're someone who likes a naked device, then you're doing yourself a disservice not getting one of those. I've had one for my Zune HD for about two years and it's held up great.
I'd give them two or three weeks or so after the GNex comes out to release the correct size.
Edit: They look much better in person than they do on the site. The actual product is more of a deep black than a dark gray.