iPhone 4 users are apparently experiencing decreased reception bars on the iPhone 4 when hands cover the external antennas. Steve Jobs revealed that the iPhone 4’s slenderness (see pictures of old 3G and 4 on the right) was due to shifting the UMTS, GSM, Bluetooth, WiFi and GPS antennas to the outer stainless steel rim of the smartphone, but that decision may be coming back to haunt the Apple's customer service department for a short while. Bars are lost when the phone is held by its bezel, because your hand is bridging the antennas located on both sides of the the bezel. However, if you slip on a cover or case, therefore killing any connection between your conductive skin and the metal bezel, the issue goes away. You can also learn how to hold the phone correctly (see in the picture above).
The video posted below isolates the problem to the lower left corner of the iPhone 4 and readers of Gizmodo were allgedly able to replicate the loss of reception on their iPhones.