Why iPhone's is plastic back?

Why iPhone's is plastic back? - The plastic back is the most obvious of the design choices. The iPad, with its all aluminum back, has seen its Wi-Fi reception radius reduced. The 3G version comes with a large patch on the top, probably big enough to provide with good reception. But the new tiny iPhone doesn't have the luxury of space: It needs to provide as much signal as possible using a very small surface. I'm sure Jon Ive is dying to get rid of the plastic back, and go iPad-style all the way, but the wireless reception is the most important thing in a cellphone. A necessary aesthetical-functional trade-off.