Ford's excellent Sync, available in Ford, Lincoln, and Mercury vehicles as an option, is one of the best mobile-to-car integration services out there. It uses Bluetooth, direct input (for mass storage devices), or some specific hookups (iPod and Zune) and then gives you access to playback options via voice control.
A video popped up today  showing the iPad's integration. Even though the tablet just came out this week, thanks to its OS similarity to the iPod Touch and iPhone, it connects via Bluetooth just fine. Unfortunately, it's not fully up and running quite yet--voice control, one of the best features of Sync, hasn't been enabled for the iPad yet, though whoever that is sitting in the driver's seat insists it'll be coming soon. I'm not really sure why you'd use an iPad over a smartphone--seems more awkward to have lying around the center console than a pocketable device. But Sync is great, and the iPad's pretty great, so it's good to see them working together well this early in the release cycle.