I hold my device resting on my fingers, not gripped in my hand. The first knuckle joint (i.e. the one closest to the fingernail) of my index and pinky fingers are aligned with the center line of the back of the device; the device is balanced on top of my fingers. This gives me the biggest range of movement over the surface of the device, and more importantly gives me better fine control over what my thumb hits - not only is thumb movement a factor, but fine finger movement as well. Adjusting the angle of holding the phone doesn't mean "shifting the device around in my hand" - it merely means changing the angle of my fingers.

I think a bigger factor in how the iPhone is better designed than most other phones is how touchscreen sensitivity drops off around the edges (if this behaviour is indeed deliberate). Before I moved to Android, I used to hold my device gripped between my fingertips and the center of my palm, but I found this didn't work as well with the Nexus One, because the sensitive edge of the touchscreen meant that the little fold of skin from my palm was creating phantom touches on the screen edge. So I stopped gripping my devices; I now hold them all resting on my fingers when I'm using the touchscreen.