Many users of the newly launched iPhone 4 are reporting problems with drop calls when their hands cover the bottom left of the phone. Apple has admitted that there is an issue with the iPhone 4 based on the placement of the antenna on the device. In an email response to a user sending a him a direct email, Apple's Steve jobs responded with this:
Gripping any phone will result in some attenuation of its antenna performance, with certain places being worse than others depending on the placement of the antennas. This is a fact of life for every wireless phone. If you ever experience this on your iPhone 4, avoid gripping it in the lower left corner in a way that covers both sides of the black strip in the metal band, or simply use one of many available cases.
Apple has adopted this response when other users complained about about losing signal on their iPhone 4.