Back when the iPhone 4 was orignally launched in mid 2010, users had complaints regarding loss of signals when holding the phone. Apparently what they call the "Death Grip" position, hold the phone in that way will make you lose signals, both on cellular and 3G. Verizon now had just launched the CDMA version of the iPhone 4 and seems like pretty clearly that the problem hasn't been addressed. See the video below to have an idea of what the 'Death Grip' is and does:

As you can see, the video clearly shows that when the phone is held in the 'Death Grip', it experiences significant loss of signals in both 3G and cellular, resulting in lower browsing speeds.

Also, it has been reported that using the phone in landscape mode, it can cause a loss of WiFi signals.