Apple CEO Steve Jobs responded to early iPhone 4 user complaints about the smartphone losing the network signal when held a certain way by saying, "Just avoid holding it that way."
Apple also released an official statement regarding the issue:
Gripping any mobile 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 sells a rubber "Bumper Case" for the iPhone 4 that costs $29, according to PCWorld.com.