It seems that some iPhone users (quite a few actually) are noticing reception problems when they hold the iPhone 4 with their left hand covering the lower part of the device. Doing this apparently causes the reception to drop significantly and there are also reports of it causing calls to drop.
There is some good news thoug. Apple and Steve Jobs himself has said that by using an iPhone bumper case it eliminates this problem entirely and you can therefore enjoy better reception with a lower chance of your calls dropping. So, if you’re having any reception issues with the iPhone 4 perhaps you should consider purchasing either the iPhone bumper case or one of the cheaper alternatives.