Online bitching about iPhone 4’s reception issue led Nikkei Electronics Teardown Squad to tear it apart to know how it ticks.
According to them, the three metal parts (Which are separated by some black slits) are all connected together. They thought their testers malfunctioned, but they tried several times with the same results. Which, after further examinations, they came up with a conclusion that the metal plates acts as a ground plane together.
They found some antenna-looking parts in the speaker module on the lower-left corner of the phone which is wired to the chassis.
They came up with the conclusion that the antenna-looking modules acts as the 3G antenna while the metal plates works as their ground.
I might explained things in a wrong way, so click the link below for more\better details.