We recently purchased a new adapter for playing our ipod / iphone in the car: the Belkin Mobile Cassette Adapter. It sounds fantastic, especially compared to the old FM adapter we used to use. Those FM mp3 & ipod adapters require finding an empty band, squashing music, sound yucky, and cost a lot more. This cassette adapter was only about $15 at Fry's. Any quality FM adapter costs a whole lot more and doesn't sound nearly as good.
This cassette adapter worked right off with our old ipod mini. For the iPhone I used a razor blade to trim away the last eighth-inch or so of rubber off the end of the plug, so it fits into the recessed iphone ear jack, and it works great.