You can see an example of the technology in action here. The DRM-protected content refuses to play, due to an external projector attached to the machine being “unauthorized” to play it. In the future, Apple says, all of their hardware will support this feature. This could make keeping and playing back DRM-protected content even more cumbersome than it is now, with people finding themselves locked in not only to a particular machine, but to a particular display as well.
As the amount of HDCP-aware content increases you can expect more people to encounter this kind of issue as well. HDCP has been a controversial technology since its introduction years ago, though to date there has been very little fallout because of it. That could soon be changing.