Apple released WatchKit today. It's their development platform for the Apple Watch. They have a great overview video on their website, and plenty of documentation.
One part of the video stood out to me. WatchKit is basically AirPlay 2.0 (or AirTunes 3.0, depending on when you started counting). With WatchKit the iPhone basically projects an app to the Watch. The Watch contains the graphics, but it's the iPhone that has the brains.
It reminded me to how you play games on the AppleTV via AirPlay. You stream the graphics to the TV, but the iPad runs the app. Makes you wonder if we'll ever see Apple TV extensions on iOS.