Eventually, there will be an iPad to Apple TV interface, and they can cut out the cable companies too.
Scary now that I think about it.
In what market and by what definition?
The analogy is not perfect, since you can buy mp3s and movies and ebooks from other services. But on iPhones and iPads the barriers to getting content from non-iTunes sources is certainly higher than it otherwise would be on a regular Mac (or any other PC for that matter).
full disclosure: lifelong Apple user