An interesting post at TechCrunch pointed out that if all iPhone 3G users upgraded to firmware V2.0.2; they problem would be resolved.

In UMTS power control is key to the mobile and network success. apparently the iPhone 3G asks for too much power from the transmitter, and the transmitter was shutting off due to overload causing dropped calls. The firmware 2.0.2 version fixes that problem of the iPhone 3G requesting too much power from the transmitter.