I had an iPhone 3G and never had a problem with the way I held my phone. The poor signal strength was one of the first things I noticed about the iPhone 4.
I just read about "how to hold" my iPhone 4. Really?! Honestly, the most natural way to hold the he iPhone (in my left hand for righthand dialing, typing) diminishes the signal. If I keep my thumb on the volume control, the black stripe on the lower left edge is smothered by the meaty part of my palm at the base of the thumb, diminishing the signal. If I hold it delicately between my thumb and middle figure, my signal is strong. This is not good design. Apparently, purchasers will need to sign up at the Genius Bar to learn how to hold their phones. Who know that there was a wrong way to hold a phone.
I have ordered a bumper. Of course, both the AT&T store and Apple Store were sold out of all bumpers ands cases in a matter of hours.
I love Apple. I've owned more than a dozen Macs, eight iPods, two iTouches, two iPhone 3G models and now the iPhone 4. Not ever product iteration has been perfect, but this is a major design flaw. I'm still working on finding a natural feeling way to hold the iPhone 4 without blocking the signal.
The rest of the iPhone 4 is great! This is the only exception ... and it's a big one.