Actual Display Resolution...?

in best for retina 1440x900 you're running at 2880x1800 but the OS is scaling UI elements to make it seem like 1440x900.

The software that has been properly retina-ized will have new hiDPI UI elements, and should show your content in real pixels, so your 1080p movie should display in 1080p. Same for Aperture. If you have an image that is 1600x1200pixels, it should display in actual pixels, so at 100% it should take up a little more than 1/4 of your screen.

 

To understand the scaling modes you have to first understand the HiDPI software option that's been in OSX (hidden) since the Lion developer previews. When HiDPI is enabled, everything gets drawn at double height and width. Apps with HiDPI support can use all of those pixels as they see fit, Apps without that support get pixel-doubled.

So if you enabled HiDPI mode on a normal 1440x900 Macbook Pro everything would be as physically large as if it were on a 720x450 display even though it is still really running at 1440x900. Retina-ready apps (and possibly parts of apps) would still display sharpley because they're able to make use of all of the pixels.

On the RMBP, the native resolution is 2880x1800. In "Best for Retina" mode, it runs in 2880x1800 with HiDPI mode enabled. So apps that are retina-ready can make use of every single real pixel since you're actually running at 2880x1800.

Apps that aren't retina ready run like they're on a 1440x900 display. It gets a bit more tricky because sometimes parts of apps (like text) can display in HiDPI mode while other parts get pixel doubled because the app isn't completely retina-ready.

The other scaled modes on the RMBP work the same way. "Looks like 1280x800" mode is actually running at 2560x1600 with HiDPI mode enabled. "Looks like 1920x1200" is actually running at 3840x2400 with HiDPI mode enabled. The final result is scaled up or down to the 2880x1800 native pixels of the display.

So in "Best for Retina" mode (2880x1800 with HiDPI enabled), a retina-ready app can display a 1080p video at pixel perfect 1080p. Apple showed this at WWDC by showing Final Cut X with the video preview in the corner displaying at full 1080p.

When you watch a 1080p video full screen, it's 1080p being scaled up to 2880x1800. Watching a 1080p video at native pixel-perfect resolution would only take up a portion of the screen since the screen has a (significantly) higher resolution than 1080p.