Well for all of those people who have waited for hours on end for your brand new iPhone 4, you may of been disappointed as there has been many reports of the iPhone 4 having signal problems. Now this only happens when you hold the iPhone in your left hand with the metal band touching your skin. And if you go back to the WWDC keynote, Steve Jobs said that the join between the 2 parts of the antenna join in the bottom left hand corner; so the contact with your hand could be short-circuiting the antenna.
Now this happens EVERY time your touch the bottom left hand corner and it slowly loses signal strength and then eventually drops the call. Not Good. See the video below.
Apparently, the signal problems do not occur when you use a case. Like the apple bumpers for example