+1 to eoc's comments.
I'm addition to that, the telstra network beats the other networks hands down when flying. I had optus then vodafone then telstra all with an iPhone 3GS. Telstra was the only one that came close to performing when in the air.
Telstra has stated that the iPhone 4 meets the blue tick standards for reception but they won't give it one because it requires the bumper or a case to meet that standard.