Michael Grothaus writing for Fast Co. Labs:

Lakamp wouldn’t reveal when Apple approached the company, but he says that once they did and iHeartRadio signed on, developing a CarPlay-compatible iHeartRadio app wasn’t a major coding challenge–mainly because Apple did a good job with the API allowing developers to add CarPlay support to their existing apps instead of having to make new, dedicated versions.

I doubt that Apple will open this to all third-party developers — likely only approaching developers whose applications would make a good fit for CarPlay. But if they did, it’s good to know that it’s easy to make your app CarPlay compatible.