I went to my local Apple store in the San Francisco Bay Area last week and saw that one of the iPhones 4's out of the 3 I tested had zero problems with signal. I could hold it any which way I wanted and no change in signal. The other iPhones I tested did have a signal problem as soon as I pressed my thumb against the buttons on the top side of them. Some members here were in disbelief of my findings and requested that I post a video so check it out. NOT All of the iPhone 4's are defective. 
http://www.youtube.com/watch?v=iTtzkZk8G-k
http://www.youtube.com/watch?v=iTtzkZk8G-k