2880x1800 and 1920x1200 are the same aspect ratio (16:10). there shouldn't be a noticeable difference

You'll mainly make the display look like poo if you try to use a different aspect ratio than your native (which is 16:10 for retina)
That's garbage. Running on anything that's not native resolution on a LCD will cause it to look blurry. Try it right now on your own LCD and run it on the next highest resolution (1280x800, 1440x900, 1680x1050, 1920x1200, 2560x1600, 2880x1800). I just attempted to downscale it from 1440x900 to 1280x800 and it looked awful.
Granted, the effects are going to be much more noticable the closer you are to your original resolution. Eg. running 2560x1600 on the new retina would look more blur as compared to running 1920x1200. The only ideal resolution that you should be down scaling to is 1440x900, as this has a perfect ratio of 4.0 pixels downscaled per pixel.
However based on my experience with iPhones and retina screens, even at a 4:1 downscalling resolution, pictures still look like absolute garbage if it's not running in native resolution. Try using any non-retina app and you'll find that it looks much worse than it does on an iPhone 3G/3GS.
IMO you're going to have a hard time running anything in Windows 7 and below at native resolution. Everything will just look extremely small as programs are not designed with retina screens in mind. This is what you can expect:
On the left is a 30" Apple screen running at 2560x1440. On the right is a 22" retina IBM screen from 2002 running at 3800x2400. See the Firefox browser in the middle of both screens? They are both the resolution, but it looks so much smaller on the IBM due to the much higher pixel density.
Here's another picture with a 30" Dell running at 2560x1440 on the left, with the IBM on the right. Based on the desktop picture, you can see how the IBM has 2.5x more space and is so much smaller.
Expect the same thing to happen if you use a Retina screen on Windows 7 and below. These monitors had a PPI of 200, whereas the new MBPs have a slightly higher PPI of 22. The good news is you can downscale the screen to a lower resolution, but it comes at the cost of pictures not looking as sharp any more that completely defeats the point of a retina screen.
Otherwise, Windows 8 is coming out soon and should also provide better support for retina resolutions
It will look exactly like non-retina MBP when run at 1440x900 resolution (same pixel size). It also might look ok when run at 1920x1200 (because the pixels are so small), although I am not sure about that.
That's what I've always believed, but now I'm not sure any more. Take a look at your iPhone 4 and you'll realize that all non-retina apps look like absolute garbage.