SpriteKit is going to be huge. Expect about a year of getting developers ready with it, and good at it, then Apple TV may have something awesome for us.
Compared to Cocos2d, I'm not that interested in Sprite Kit as a framework. But combined with the new gamepad APIs, I think you might be on to something in regards to the Apple TV.
Interesting suggestion - I was surprised to see such a direct competitor to cocos2d, and despite the development budget of Apple I'm a little skeptical of their commitment to building a game engine. Maybe it'll form the basis of a better set of game engines. I'm definitely skeptical of their physics engine over Box2D.
Games tend to be the most likely apps to be ported between systems, so the appeal of a single OS API seems limited to me.
What's SpriteKit? Sugar Cube Software's Cocoa Sprite Kit? How is it related to iOS7?
No, it's a new iOS 7 / OS X framework. Since it's an API, I think the only real discussion is in the developer docs, which are NDA'd.