While Apple continues to claim publicly that the only thing wrong with the iPhone 4 is the software that determines how many bars of signal strength are displayed at a given time, there are some who have purchased an iPhone well after the launch that seem to have no problem with the call dropping and loss in signal strength seen on the first batch of handsets. The fourth and fifth numbers of the iPhone serial number refer to the week that the phone was produced. There is some speculation that units birthed after week 27 had been put through a slightly different manufacturing process that allowed for a non-conductive coating to be placed over the area where the phone's two antennas left a small gap. The theory is that when the phone was held in this spot, the user's body created a "bridge", conducting the signal and de-tuning the antennas. By coating the phone in that spot with a material that cannot conduct the signal, the reception problems should end.
If you received an iPhone in the last few days, check the serial number to see if it is greater than 27. If so, get a hold of a multimeter to test the impedance of the metal frame. Hold the two sides about an inch apart near the gap in the frame (see the picture below). If the reading is much greater than 1 ohm, than your iPhone 4 could have the coating as a non-conductive coating would have a high resistance. The last thing you need to do is to report back to us because if indeed Apple has changed its manufacturing process to add non-conductive coating, it would be proof that their public hard-nosed stance is a cover-up and privately, the Cupertino based firm knows they made a boo-boo.Apple iPhone 4 Specifications