According to iFixit, the guys who always do a great job in disassembling and explaining Apple hardware, the iPhone 3G does NOT feature the Broadcom chipset mentioned before. This comes as a surprise, as also Business Week confirmed the Broadcom A-GPS to be inside the iPhone.
Instead, the iPhone was equipped with an Infineon Hammerhead II PMB 2525 chipset. Beside being from an other manufacturer, the Hammerhead II features basically the same as the firstly assumed Broadcomm, including "standalone" mode (which doesn't seem to work in iPhone 3G) and -160 dBm sensitivity, which should make it more sensitive than a SiRF III, but doesn't.
I'm curious where the antenna was hidden... let's just hope the guys from iFixit will discover it soon ;)