The iPhone Death Grip is somewhat real, but it's more subtle than a lot of people have been putting on. It's not a deal breaker and it's not a reason -- by itself -- not to buy the iPhone 4. But the nearly hysterical online reaction to the death-grip news reveals what people are really thinking.
First, Apple has issued an official statement, with which I completely agree.
"Gripping any 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," Apple said. "If you ever experience this on your Phone 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."
If I hold the phone in a slightly sweaty left hand, with my fingers covering the three black lines on the phone's edge and the bottom left corner in my palm, signal strength is somewhat reduced. If I had to pick a number out of the air, I'd say it's by 3 to 5 decibels per milliwatt (dBm). Feel free to correct me if you have the appropriate lab equipment. The hand involved has to be a little sweaty to encourage conductivity, or the trick might not work.
This doesn't have any effect on connecting voice calls in areas with a strong signal, but it can make the difference between connecting and not if you're already in a fringe signal area.