That would be with XP style scaling. On WinVista and later that mode can only be used with scaling levels less than or equal to 125%. After 125% you get Vista style scaling, which depending on how well behaved an application is will result in one of two things.
If the application is flagged as being DPI scaling aware (Office, web browsers, etc) then the application will take care of scaling on its own, and hopefully render a suitably large image natively. If an application isn't flagged as being DPI scaling aware, then Vista reverts to "fractional scaling", where it simply does a bilinear upscale of the application window, resulting in a blurry, god-awful mess where nothing was rendered natively.
Apple does something similar here, but their innovation was that instead of resorting to fractional scaling on non-aware applications they do integer scaling, which is far cleaner in practice. Furthermore all of Apple's drawing APIs were retina aware, so applications that weren't fully retina aware themselves could still have their text drawn natively, whereas Vista would always have to upscale the resulting Window.
The worst case scenario then for Mac OS X (a non-aware application not using Apple's drawing APIs) is that at the default 2x setting (backing scale factor 2.0) every element will simply be scaled up by a factor of 4:1; every 1 pixel now occupies 4 pixels. This means that there aren't any benefits gained from using the retina display, but using integer scaling means that this doesn't introduce any fractional interpolation artifacts that hurt the text quality, since every original text pixel maps cleanly to 4 display pixels.
Right now the expectation is that Microsoft will be introducing something similar in Win8.1. There's only so much they can do without breaking backwards compatibility, but if they follow Apple's "render big then scale down" philosophy rather than Vista's "render small and scale up" philosophy, then results should be much cleaner.