Jason Snell reviews the new MacBook:
The MacBook’s retina display is physically 2304 by 1440 pixels. In standard “2x” retina mode, that means the 12-inch display should look like a non-Retina display at 1152 by 720 pixels. While that’s an option in the Display preference pane, by default the MacBook displays at a scaled resolution that’s the size of a 1280 by 800 pixel display, more or less the same amount of real estate as an 11-inch MacBook Air. That’s also the resolution that the 13-inch Retina MacBook Pro displays at, but on that screen it’s the native 2x resolution—here, the MacBook is drawing a 2560 by 1600 pixel interface and then scaling it down to fit in 2304 by 1440 pixels.
I’m surprised that the new Macbook doesn’t ship with the native resolution set by default. My guess is that it might be related to LCD panel cost, but either way it seems like a smart move to scale it down — working with less than 800px vertical display pixels feels cramped.