"A software origin for the hand position bug, nicknamed the "death grip" for its tendency to kill the cellular signal if the antennas are bridged, would explain how some have replicated the problem on the iPhone 3G and the 3GS. As iOS 4 changed the antenna's priority from the strongest possible signal to the most reliable, it may have changed that behavior for all phones, not just the iPhone 4. Owners of the 3G MicroCell also support this through their lack of trouble; since the femtocell is always the most powerful cellular signal when nearby, iPhones always stay connected. The same behavior may likewise explain why some without MicroCells haven't had success replicating the problem, as any attenuation of the signal might not be enough to confuse the phone if the user is close to a good cell site."
DJRumpy writes: It appears that iOS4 may have changed the signal priority of the iPhone to look at a more reliable connection rather than at signal strength. This would explain why 3G and 3GS are also seeing the same issues being reported about the 4G signal strength: