first of all let me say that I am not from Microsoft. I am an independent developer (MVP) that has a strong passion for WinRT and many other MS technologies.
The "Metro" platform was created only for user applications and not for administrative purposes. This means that you don't have UAC nor any kind of management such as event viewer or device management.
Every administrative application typically require an elevated token (administrator) and consequently must run in the dear old desktop. This means that you can't use WinRT to develop drivers installers.
For your info, please note that a portion of WinRT API can be used from desktop applications such as Windows Forms, WPF or MFC.
As far as I know there is still no info on what will be available for the "Windows RT" devices (the ones the come with ARM CPUs). I suspect that only OEMs will be able to install drivers on that platform (and this does make sense at all).