I have a BMW M3 and i use the BMW iPod integration kit. It works AMAZINGLY. problem is it is only for BMW's.
These guys make a better one, i dont know which other car's they make it for.
http://www.dension.com/main.htm
Alpine also sells a kit to hook up to their decks.
here is a pic of what mine looked like in my old car, in my M3 i moved it by the hand brake so i can still talk on the phone and once im done with my call, the music comes back on. it is the perfect hook-up for iPhone/iPod car integration.