+1 for the Innocase Surface.
It's the first and only case I bought for my N1. It's quite sturdy, and I feel it adds just the right amount of thickness. The coating on the case adds some nice traction too for gripping. The naked N1 is a bit too thin and slick for me to feel safe holding by the edges when on a call.
Only taken one significant fall so far, about 3 feet onto concrete. The case did it's job, the two pieces coming apart but absorbing the impact. Only ended up with a tiny ding on the power button. All in all I'd highly recommend it.
Sent from my Nexus One using XDA