Microsoft today detailed the various display resolutions supported in Windows 8. Proper resolution support is needed in the OS to offer customers a broad choice of form factors while having predictable user experience. Also to make developers easily build apps that look great on all form factors in the Windows ecosystem. Also Microsoft has chosen 1024×768 as a minimum for Metro style apps. The Metro-Style apps will run great on screens up to 2560×1600 resolution.
You can read more about this in detail at Building Windows 8 blog.