EDIT: Looks like Fineance beat me to it
At the native resolution all the interface elements become really small and for the majority of people the computer isn't usable. Here's a screenshot of my 15" rMBP running at the native 2880x1800:
http://i.imgur.com/IZ0c389.png
Notice how text is too small to easily read?
To solve this Apple has traditionally mapped four physical screen pixels to one software pixel (a point). This halves the effective resolution in each dimension (to 1440x900 on my 15" rMBP) but means the on-screen elements such as text, graphics, etc appear very clear and sharp.
As the pixel density is so high on retina displays you don't have to map four physical screen pixels to one software pixel in order to improve the picture quality over standard displays. Doing it this way gives the best picture, but you can sacrifice the quality a little and use software scaling to allow larger effective resolutions (1680x1050 and 1920x1200 on my 15" rMBP).
This works by rendering the on screen image at double the effective resolution and then scaling it down to fit on the retina screen. For example, if I run my 15" rMBP at 1680x1050 the OS will actually ask the graphics processor to render the screen at double that in each direction, so 3360x2100. It will then scale that down using software to the 2880x1800.
The retina MacBook does the same thing as my rMBP. The best resolution would be 1152x720 as standard. The problem is that doesn't provide a lot of usable screen space, so instead Apple uses the software scaling to run at 1280x800 by default.
The 1440x900 you've seen is another supported scaled resolution and works in the same way. It won't be as crisp as 1152x720 would be but it's still much, much better than the old displays and the on screen elements aren't too small to be usable.
AnandTech has a good explanation in the original rMBP review:
http://www.anandtech.com/show/6023/the-nextgen-macbook-pro-with-retina-display-review/6