Anand Shimpi and Brian Klug over at Anandtech have done some rigorous testing of an iPhone 4 alongside an iPhone 3GS and a Nexus One. His findings, not surprisingly: the Death Grip is real, it's serious, and you need bumpers. The test results unambiguously replicate what has been effectively crowdsourced over the past week, and should increase the pressure on Apple to stand up and deal with the problem. (I'm not saying it will, but it most definitely should.)
I'm an RF engineer by trade, and when I noticed the bars dropping on my new iPhone I wanted to try and quantify it a little better. So the first thing I did was key in *3001#12345#* to enable field test mode so I could see real RSSI (signal strength) and SNR (signal to noise) numbers. Nothing. As it turns out Apple yoinked FieldTest.app from iOS 4, so all we've got is the vague, slow-to-resolve bars readout. Thanks, guys. Given that no matter what, the bars on an iPhone 4 had to represent something different than what they did on my iPhone 3G, there was no real way of knowing what the drop in bars represented in real signal strength.
The Anandtech guys hit on a great solution for showing signal strength on his iPhone 4: having modified his 3GS to show RSSI instead of bars, he restored his iPhone 4 from the 3GS backup, and the modified status config came along with it. Using that, he was able to conclusively determine what the effect of the Death Grip was on signal: up to a worst case of -24 dBm attenuation, and on average -20, which will take you from four bars to one or none. Using a case, the drop was -7 dBm, same as the Nexus One.
Go to the Anandtech article if you want the details, it's a fascinating read if you're a radio geek, or if you just like to see how people who know what they're doing go about figuring things out. He did find that the iPhone 4's antenna is much improved, especially in low signal conditions. However, his conclusion is as clear as can be:
At the end of the day, Apple should add an insulative coating to the stainless steel band, or subsidize bumper cases. It's that simple.