Many users have been complaining that when they hold the iPhone 4 in their left hand, the reception bars go down completely. They can still make a call, but the bars look like they have no reception.
A YouTube user has posted this video which attempts to show the same thing happening on an iPhone 3G running iOS4. It looks like this could be a software bug instead of a hardware design problem. Great news for Apple and all their fans if it is true. Let's hope they release a patch very soon.