The TiPb store just got the new Case-Mate Barely There Case for iPhone 4 in stock, so I decided to take a quick look at it. As you'd expect by the name, it's super thin and doesn't add any perceivable bulk to iPhone 4. Same token, while it will protect against scrapes and bumps, there's no padding to help you out in case of a high fall.
The finish is soft-touch but I don't find it that to be a huge difference from iPhone 4 by itself. It included a screen protector so you can be covered front and back.
Is it enough to stop the de-tuning that occurs when you have a low signal and touch the bottom left hand side of the antenna band? We're going to test that out and we'll update with the results asap.
UPDATE: Added a picture at the bottom taken with the iPhone 4 flash while the case was on. No distortion or flare that I could see, but the cutout is fairly wide around the whole camera assembly.