I haven't seen a retina Macbook Pro, but I have followed this thread with some interest. MDomino proved quite convincingly that scaling from 1440x900 to 2880x1800 is done by pixel doubling for applications that don't yet understand to take advantage of the new screen. However, it is evident when zoomed in that some pixels within fonts that are supposed to be black actually have color, which to me is a sign of subpixel anti-aliasing. When a font with this kind of anti-aliasing is made without pixel-doubling, i.e. natively, it improves the quality of the font shapes (although some people still see the color "fringing" and don't like it). In "native mode" subpixel antialiasing utilizes the red, green and blue subpixels of each physical pixel to create higher-quality fonts. (Google subpixel anti-aliasing to understand how this works.) After pixel-doubling, there are no longer any subpixels (the 2x2 size "zoomed pixel" is not split up into a red, green and blue part, from left to right), and subpixel antialiasing makes no sense.
Right now I'm not at my Mac, so I don't quite remember which settings are available nowadays, but there used to be anti-aliasing settings in the Appearance section of System preferences. At least in the past, some (or at least one) of these settings would not use subpixel anti-aliasing, but just gray-scale anti-aliasing. It would be interesting if someone with a retina MacBook Pro could test if this setting still exists, and if they think that the quality of text on pixel-doubled becomes better when anti-aliasing is not done using subpixels. I think it should.
If I am correct, pixel-doubled grayscale anti-aliasing of text should look much better than pixel-doubled subpixel anti-aliased text, but it would not look as good as subpixel anti-aliased text without pixel doubling on an equivalent non-retina screen (1440x900). So, apps that don't take advantage of the retina display would still look slightly worse on the retina display than when it is run at "native" resolution, on the old 1440x900 display.