Apple has just responded to the many bad feed backs about the iPhone 4 signal problems, saying that its due to the fact that your not holding it correctly.
Just like the older iPhone, there is a a metal band in the back that is both a 3G and wi-fi receiver, and the same goes for the new iPhone, so if you hold it and touch both parts of the antenna, it severely reduces the signal, and maybe even drop your calls.
“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,” they said.
“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.”
If you want to avoid this issue, just simply don’t hold your iPhone that way, or you can also buy a case that stops the skin contact with that certain part of the phone.