The thinnest case on the market is the invisiShield. It is an ultra-thin film that covers all of the flat parts of your phone and is slightly tacky to the touch. Check out these links:

Weedwhacked: YouTube - How to Weedwack your iPod Nano

Slight Grip: YouTube - invisibleSHIELD Grip Test - iPhone on the Freeway

Screws: YouTube - iPhone Case Review at CES

And here is their website: ZAGG | invisibleSHIELD | Cases, Screen Protectors, Covers, Shields, Skins, Invisible Shield

I hate bulky cases/protectors, so this really knocked my socks off when I got it installed. Check out the videos and customer testimonials. Every friend I have recommended it to has been pleased with the results on their device.

ZAGG | invisibleSHIELD | Palm Centro Cases, Screen Protectors, Covers, Shields, Skins, Invisible Shield