This is something that has already been mentioned but I guess I'll post my personal experience:
I did a test today where I took my iPhone 4 (which suffers from the "incorrect holding position" syndrome) to a high signal location, and I couldn't get the bars to drop for over half hour of testing. I then took someone else'd iPhone 4 and tested it at my home location. The signal dropped at my home in a matter of seconds on both of the iPhone 4's.
I tried restarting the phone, and each time the signal at my home would drop, and would remain full at the new location.
I did a test today where I took my iPhone 4 (which suffers from the "incorrect holding position" syndrome) to a high signal location, and I couldn't get the bars to drop for over half hour of testing. I then took someone else'd iPhone 4 and tested it at my home location. The signal dropped at my home in a matter of seconds on both of the iPhone 4's.
I tried restarting the phone, and each time the signal at my home would drop, and would remain full at the new location.