For Windows 8 on x86 32bit and 64bit, Microsoft brought back the Windows XP days: there's Windows 8 and Windows 8 Pro. Windows 8 Pro includes, on top of Windows 8's base features, "features for encryption, virtualization, PC management and domain connectivity". On top of that, Windows 8 Pro users can install the separate Windows Media Center pack. And praise be Fiona: on-the-fly language switching comes to all versions of Windows.
The ARM version of Windows 8 will be called Windows 8 RT, and will not be sold separately, but only pre-installed on ARM devices. This gives Microsoft quite a bit of control over the ARM experience, including mandating that no other operating systems be installable on the system. Windows RT will focus on Metro mostly, but will also include the traditional desktop - albeit without the option of installing additional classic applications. Microsoft Office comes standard on Windows RT.
Lastly, there's a version you'll only see at work: Windows 8 Enterprise. "Windows 8 Enterprise includes all the features of Windows 8 Pro plus features for IT organization that enable PC management and deployment, advanced security, virtualization, new mobility scenarios, and much more," Microsoft details.
All in all, this looks like a sensible set of options, although I'm not too keen on making Windows RT as special as it is. I had hoped - against my better judgement, of course, I'm nothing if not completely jaded and cynical - that with Windows coming to ARM, we'd see an x86-like freedom and openness coming to ARM, so we could build our own ARM systems like we can with x86 machines today.
Not that any of this matters in the end. People will get Windows 8 when they buy a new PC, forced into an interface that just hasn't been designed for a mouse and keyboard. It's going to be a bloodbath.