The new Retina MacBook Displays may have an incredible resolution of 2880 x 1800 pixels, but OS X doesn’t treat it as such. Instead, it treats the display as a 1440 x 900 HiDPI display in its default configuration, meaning that while text, video and images may look crisper, you don’t actually get 2880 x 1800 pixels worth of desktop space.
In the Displays panel in Systems Preferences, you can tell your Retina MacBook Pro to give you more desktop space, up to the equivalent of 1920 x 1200, but that’s as far as it goes. What if you want a 2880 x 1800 desktop, though, with each pixel mapped one to one? There’s an app for that.