AnandTech’s not-quite-exhaustive but extensive four page analysis of Apple’s antenna problem using iPhone 4, iOS 4.0, iOS 4.0.1, and iPhone 3GS:
Using a bumper the iPhone 4 behaves no differently than the 3GS. Hold the phone as tightly as you want with a bumper and it’ll lose as much signal as a 3GS or Nexus One. Put some sort of insulating coating on the stainless steel band and you’ll significantly reduce, but not eliminate the issue.
And regarding iOS 4.0.1:
The new signal strength visualization in iOS 4.0.1 is simply going to be more honest with iPhone users.
And iPhone 4 overall:
The phone itself delivers better battery life than anything else in its class, has good performance and a wonderful screen. Whether or not the antenna design manifests itself as an issue really depends on AT&T’s coverage where you’re using the phone.
In other words, iPhone 4’s attenuation problem is only a real world problem when signal strength is ultra weak anyway. Otherwise, put a rubber on your iPhone and practice safe signal reception.