The Windows 8 user interface is designed to scale to systems of all sizes. Like Windows versions of old, it will have to scale all the way from 1366×768 10-inch tablets up to 2560×1440 30-inch desktop monitors and beyond. But it's not just different numbers of pixels that Windows 8 will have to cope with: different sizes of pixels matter too. Windows 8 will have to scale from screens with around 96 dots per inch all the way to screens with almost 300 dpi, as system vendors are finally starting to increase pixel densities (no doubt inspired by the launch of a rather successful new tablet). An explanation of how this has been done is the subject of a new post on Microsoft's Building Windows 8 blog.
Windows has long had support for both different resolutions and pixel densities. The former are easy to handle; run Windows on a system with a high screen resolution and you'll fit a lot more stuff on screen. Traditionally, monitors have all offered about 96 dots per inch, so no matter what resolution you used, the objects on screen (buttons, text, images, and so on) have more or less maintained their physical size.