The clever guys over at Anandtech.com have published a nice illustration of how the iOS 4.0.1 signal fix works. As you can see the biggest change is within the 5th bar of signal strength. It goes down to the 4th bar much fast and it takes a bit longer before it goes from 1 bar to ‘no service’. Read the in-depth analysis over at Anandtech:
The iPhone 4 Redux: Analyzing Apple’s iOS 4.0.1 Signal Fix & Antenna Issue [anandtech.com]