Up until just a few months ago, I had always used an FM transmitter like this Belkin one to play my iPhone through my car stereo. It was unwieldy and it made my Apple Lossless-encoded music sound like garbage, but at least it (kinda) worked for those long trips. If you are still using transmitters but are looking for a wireless solution in your car, Engadget has the scoop on the new TuneLink Auto from New Potato Technologies. It plugs directly into your cigarette lighter and provides a Bluetooth audio link between your iOS 4.0 or higher iPhone, iPod touch or iPad and then transmits it to your radio via the FM station of your choosing. This way, once you pick your music or start the iPhone NPR stream during your morning commute you can put it anywhere in the car and not have wires dangling all over the place. The front of the device even has a USB port that can charge any electronics you may have taken with you.
The TuneLink Auto is controlled by a free app you have to download from the App Store. According to New Potato, using the device is as easy as "PLUG IT, PAIR IT, PLAY IT, SHARE IT!" and is available now for US $99.99. If it works better than the FM transmitters I used for all those years, it may be worth the money. But if you have the option I would definitely go with a wired connection of some sort, as the purpose of having your music in the car is to enjoy it -- not to constantly be changing stations and adjusting wires just to hear it at all.