It's going to work the same way it technically works on the iPad. UI elements are pixel doubled. Photo and video assets are rendered doubled unless they are told to render at 50%, although on the desktop this will probably be a bit "smarter". You get the real estate where you need it, but not at the cost of usability.
That's what actually happens on an iPad. A 200x200 image shows up in 400x400 pixel space unless you tell it to render at 50%.
On the desktop you have the luxury of just "zooming out". If you are in a browser for example, you can just zoom out. If you do that right now, you'll notice that it becomes unreadable pretty fast. But on a higher DPI display you can zoom out further and still have everything be legible.
That's all I needed to know. I know how Apple's current retina displays work, I was asking based on the "rumors" that were leaked earlier on in the year, that suggested you could change the resolution to provide more (non-retina) screen space, or choose a crisper, pixel doubled resolution (with less physical space). I have my answer.