The Vapor case is a Faraday cage
I have a Vapor case. It's made of metal. When I ordered it, I wondered how they could wrap a metal band around the iPhone 4 antenna and not kill the signal.
Guess what? They can't.
When I put the Vapor case on my iPhone 4, it kills 2 bars of signal unless I am in an area with super-strong signal. If I have 4 or fewer bars, it knocks off 2 (and sometimes 3) bars.
It's a particularly bad problem because of the hex nuts: I was in New Hampshire the other weekend, where AT&T coverage is weak, and I was getting only 1 or zero bars the whole time; since I didn't have a tiny hex key with me, I could not remove the case. My wife's iPhone 4, which has just a rubber case, was fine with 2-3 bars.
I love the look and coolness of the Vapor case, but I don't really understand how they can sell them given this problem.
(I'm going to return mine, the nice folks at Element Case have a fine return policy)