And so it begins... I believe you mean Windows RT [1] and not WinRT [2]. Both x86 and ARM processors can use WinRT (metro interface) however Windows RT is the flavour of windows that only runs on ARM processors and only supports third party apps using WinRT [3].

[1] http://en.wikipedia.org/wiki/Windows_RT [2] http://en.wikipedia.org/wiki/Windows_Runtime [3] http://www.i-programmer.info/news/177-windows-8/4079-three-w...

From the wikipedia article:

  Windows RT (RT stands for Runtime)

...

  Not to be confused with Windows Runtime

Great choice of names, there.