[ame]http://www.youtube.com/watch?v=vPJ0AWAFCJI[/ame]
YouTube user pdye723 has posted a video testing the signal loss due to the way you hold the iPhone 4. Earlier this week, Steve Jobs suggested to the people experiencing the signal loss, to either hold the device differently or buy a case:
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.
Well after watching this video(and many other reports on the web), using any Bumper-like case will fix your signal problem. For now, Steve's solutions or waiting for the rumor of the 4.0.1 signal fix to come out is are the only way to go.