Just read that as well, which makes it a very valid option. Would be great to develop on a screen like this.
No, but if you want you can get 11" or 13" ultrabook from Asus with a 1080p resolution.
Except that Windows doesn't have the HiDPI concept, which makes all your UI truly tiny and useless.
You can change the DPI in Windows. Not all apps properly support high DPI settings, but that probably won't be different on OSX, at least in the beginning. What you can't do in Windows is have different DPI settings for different displays, and I wonder how a retina MacBook will work with a non-retina external display.