Under the hood, iPhone OS is virtually the same as the OS X that runs on Mac desktops. Steve Jobs announced this back in 2007, and it has been confirmed by any number of jailbreaks that allow third parties to explore the iPhone OS from the command line. It may be closed but iPhone OS is still basically OS X.
What differentiates iPhone OS from Mac OS has been the openness of the platform. Developers must go through Apple's review process to deploy software to the vast audience of iPhone OS device owners. Apple sets the rules, ensures the quality, polices the system. It's a new way of computing, closer to the TiVo or Wii experience than, say, the Windows or Linux experience.
Could it be the future of a new Macintosh line?