I've been trying a 'naked' iPhone 4 - no Bumper or case - with a Telstra SIM. Deliberately holding the phone in the notorious left-handed death grip did cause a slight reduction in displayed signal strength, but the only time I managed to force a dropped call was when I rang someone from the back corner of a supermarket on the lower level of a shopping centre - a location where my own phone can't make calls at all.
But the death grip doesn't come naturally to me, as it involves holding the iPhone so the lower left corner pushes into the fleshy part of the palm. My 'normal' left-hand grip is higher up the phone so that the microphone end of the handset is near my mouth, not below the joint of my jaw.
Since I'm a southpaw, the most natural way of holding a phone is in my right hand, and then none of my fingers are anywhere near the gap between the antennas.
So my conclusion on the death grip is threefold. Firstly, it really does exist, just as it does on all the other phones I've tried.
Please read on for points two and three, plus my observations on the proximity sensor.
Thirdly, if the death grip comes naturally to you, then the geometry of your hand must be very different to mine, and I suspect you're one of those people who insists on shouting into their phone in public places rather than holding it closer to their mouth.
In line with reports from various Australian reviewers, I found the death grip has a more immediately noticeable effect on data speeds than it does on voice calls. But I really had to go out of my way to apply the death grip when using the iPhone 4 in either portrait or landscape orientation, so for that reason I regard it as a complete non-issue for data.
And as for the proximity sensor (which is used to disable the touchscreen and backlight when the iPhone is held to the ear), all I can say is that the iPhone 4 that Apple lent me works perfectly well in this regard. But you'd expect demo units to be individually checked before dispatch, that only proves that it isn't an inherent design flaw affecting every unit. Still, I'm yet to hear from an Australian iPhone 4 owner that's experiencing the problem, which tells me that it's limited to certain production runs, or to a small proportion of iPhone 4s (which could be the same thing).