Apple is following in the footsteps of the Nexus One by using dual microphones to help improve audio quality and suppress background noise.

Interestingly, the two microphones are positioned at the two far ends of the phone. The microphone at the top of the iPhone is most likely utilized for capturing background noise to be analyzed by internal circuitry and used to cancel any noise put into the main microphone other than the user's voice.

You can also see the ambient light sensor which adjusts the iPhone's brightness as lighting conditions change.

The proximity sensor sits right next to the ambient light sensor. This will turn the display off when you bring the phone up to your ear thus preventing any touch interaction while it's pressed up against ear.