I got one of these fitted to my E46 BMW 330Ci yesterday......
http://www.diceelectronics.com/Merch...=DICE_IPOD_BMW
They do Mazda too.
I've been looking for some kind of iPhone car integration for a while - my car doesn't have the built-in interface the newer cars do, and FM transmitter was rubbish - although Dice don't officially state compatibility with iPhone, I had read that the two worked OK together. Its supposed to allow display of album/artist/song/playlist info on the car's display, and support steering wheel controls too.
It does work, but......
It can display info on the car's screen, but not in enough detail to allow music selection. The iPhone displays 'Accessory Attached', so the screen is unusable in iPod mode (everything else works fine), so music selection is pretty random.
If you switch the iPhone off, connect it to Dice, then switch on, the iPod display works fine for music selection, but you can no longer see any info on the car screen, or use steering wheel controls, although music playback is fine. In fact, better than fine, very good indeed.
I'm still fiddling with it to see if I can improve usability, but controlling music from the iPhone itself does not cause me a problem, although it would be nice if I could forward/rewind tracks from the wheel. I'm also going to contact Dice to see if they can help.
Ciao
T