SC,
If you're pikey like me... get the standard wired headset/mic combo, remove the ear buds and strip down the wires, purchase cassette adaptor/3.5mm adaptor to plug into your car stereo, remove the headphone jack on that, connect the 2 together and use liberal amounts of speed tape to attach wiring to your dash. Then add cheap phone cradle courtesy of halfords (note other automotive stores are available) and you get a factory fitted handsfree kit for a tenner. And in to the bargain, you can play mp3s etc on your car stereo from your phone (assuming your phone is capable of playing them!)....
pikey but it works!