Something that maybe not everyone has thought about with the iPhone is that the microphone is good enough to use in conjunction with an FM transmitter in the car as a speaker phone. This is a cheap and secure way to drive and speak at the same time.
The quality of the sound for the receiving part may not be the best, but it's good enough and it's better than using a headset. It even stops the music when the phone rings, such as a good installation of a phone kit for a car will do when receiving a call.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080722161702202