Apple's iOS 7 brought a whole new raft of features with it to iPhones, iPads, iPods, and other various things. One of those features is iTunes Radio, and if you want to use it to its full capacity, you'll need a Nissan.
Nissan says its new cars will be the exclusive launch parters for iTunes Radio and have "unique iTunes Radio materials," which I believe means they'll have a more thorough integration of the service in their cars than others will be able to get by streaming iTunes Radio through an iPhone or iPod.
Nissan wants to be in on the ground floor with Apple's newest service, and can you blame them? Many of Apple's past products have become ubiquitous in the marketplace, so why not take a risk and get in on the ground floor with their newest offering?
Makes sense to me. If it fails, it fails. If it succeeds and they didn't get in at the start when they had a chance, they'd kick themselves repeatedly.