The guys over at Engadget contacted Apple over the reception issue when you hold the bottom left side of the iPhone 4. Apple response was:
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.
So there you have it; change the way you hold your iPhone 4 or buy Apple's(or any other) iPhone 4 case, Bumpers. I, personally, don't seem to be having any reception problems, but I've heard putting a case on it fixes the problem.
Source:Engadget