By now, just about everyone in the U.S. has an Apple iPod, or one of the many competitors. Most automakers have rightly realized that allowing people to use their iPods in the car with a well integrated interface would be a big selling point. The best system I have tried is Ford's Sync, which will talk to an iPod (and most other mp3 players, as well as flash drives). I have also heard good things about Chrysler's MyGIG system. GM is offering it's PAL system on many of its vehicles, and it will eventually become ubiquitous.
I was looking into the new Mazda6 recently, and I was amazed that Mazda does not offer any digital iPod integration on this car. You can get a dealer installed add-on kit (for a hefty price), but all you get out of the box is the analog aux input. This is a real turn-off for me, in fact it is nearly a deal-killer.
Mazda just released a major update to the Mazda6, the should have put iPod integration at the top of the new feature list. Sure, bluetooth audio streaming is nice, but most people don't use it yet, and Apple doesn't support it yet.