ZDNet recently published an article called: "iPod your car, the right way" by Jason D. O'Grady.

They indicated discussed the several ways to connect your iPod to your car's stereo, ranging from simple and poor sounding to complex and high fidelity.

The article mentions FM transmitters, cassette adapters, auxilary inputs, direct integration, and in-car audio solutions. It's a good read, so go check it out here!