AnandTech have done some more scientific testing of the problem and come up with a few interesting results. Turns out the iPhone 4 actually performs slightly better inside a case than a phone like the Nexus One, which has had similar issues crop up, but it's slightly worse when held in the hand, reporting an average signal drop of 20dB. Here's where it gets a little wacky, though: the signal meter in iOS 4 uses a highly compressed range, so that 20dB drop can either leave you looking steady at five bars or drop you all the way to zero, depending on what the actual signal level in the area is. The range of values between one bar and four spans just 23dB, while the range for five bars is 40dB. That means holding the phone in an area with a strong five-bar signal will have no apparent effect -- you can lose 20dB at full signal and still see five bars -- but holding the phone in an area with weaker coverage will easily drop the meter to one bar, since the 20dB signal drop covers almost the entirety of the remaining 23dB scale.
Of course, that's just the on-screen display, which Apple can and likely will tweak in a future iOS update. The real question is whether the reported signal has anything to do with performance, and Anandtech agrees with our general experience, saying that the iPhone 4's improved signal to noise ratio means it actually does a better job of hanging onto calls and using data when there's low signal than the iPhone 3GS. In their words, "this iPhone gets the best cellular reception yet, even though measured signal is lower than the 3GS." However, there's no getting around the fact that we've definitely dropped a couple calls with the iPhone 4 by holding it the wrong way, and Anandtech says the only real solution to the antenna issue will be for Apple to either subsidize free bumpers or add an insulative coating to the antenna band. We'll see what happens -- we've got a feeling an iOS update is on the way.