Yesterday Microsoft has released full details on Windows on ARM (or WOA).
According to Sinofsky's post, Windows on ARM is a new member of the Windows family that builds on the foundation of Windows. It has lots of stuff in common with Windows 8 and will enable a new class of PC with unique capabilities and form factors, supported by a new set of Microsoft partners that expand the ecosystem of which Windows is part. WOA PCs are still under development and the main target for Microsoft and PC makers is to ship them when Windows 8 for x86/x64 PCs will be released. These PCs will be built on innovative hardware platforms provided by NVIDIA, Qualcomm and Texas Instruments with a common Windows on ARM OS foundation.
WOA will have the Windows desktop, with familiar apps like Explorer, Internet Explorer 10, the next version of the Windows Live apps and Office 15 (Word, Excel, PowerPoint and OneNote): those apps have been significantly architected for both touch and minimized power/resource consumption. Everything else will be Metro-style apps.
All Metro-style apps will run on WOA just like you would expect: those are apps written using the new Visual Studio 11 tools in HTML5, VB, C# and XAML - or in C/C++ if developers prefer (that's the language most Windows programs are written in, though you can't just turn an existing x86/x64 Windows app into a Metro app), leveraging WinRT, the new Windows APIs for building Metro-style apps. Consumers will obtain all software and device drivers through the Windows Store and Microsoft Update/Windows Update.
The Windows Consumer Preview (the beta of Windows 8 on x86/x64), will be available for download by the end of February. This next milestone of Windows 8 will be available in several languages and will be open for anyone to download. Around this release, a limited number of test WOA PCs will be made available to developers and hardware partners in a closed, invitation-only program. These devices will be running the same branch of Windows 8 on x86/x64, but they will not be samples or hints of forthcoming PCs, just development tools for hardware and software engineers running WOA-specific hardware.
You can read more here, from Building Windows 8 blog.