1.x86 and arm architecture are different and it is not easy to port. 2.No existing desktop application for ARM.
Windows RT seems to be windows 8's partial port for ARM chips as I understand.
The only difference between Windows RT and Windows 8 is that the only sanctioned way to install software on Windows RT is from the Windows Store, where all apps run on both CPU architectures.
From the OP quoted article:
"While Windows 8 and Windows RT can both run so-called “Modern” apps (formerly known as “Metro” apps) designed for touch-sensitive screens, even those apps must be converted, or ported, to Windows RT, and not all Modern apps appearing in the Windows app store have been ported over, compounding compatibility issues."
If you write a .NET/XAML app, by default it will be compiled to a CPU neutral IL that runs on all architectures (you can disable this and target a specific CPU architecture).
If you write a C++/XAML app, by default Visual Studio will compile different versions of the app for each architecture.
For the vast majority of Win8 apps, "converting" to ARM is just a cross-compile. It is only when you are doing very specialized things that you actually need to worry about the CPU architecture your app will be running on.
Also in the Linux world they are working on improving the performance and battery efficiency with kernels 3.7 and 3.8, and they will keep improving them in future versions, as they are only at very early stages of ARM support. I think the 64 bit ARMv8 chips will get great support for Linux from day one, when they come out in 2014, as support for them it's already being baked into the kernels right now.
?!?
I'm a huge linux fan but I've never heard of this outside of some sort of processor-emulation layer and full VM which wouldn't give you the world's best performance.
Have I missed something?
--edit--
Did you mean support for older apps is (comparatively) easy because they can be rebuilt from source?
Also I'm not really sure what you mean by linux support for ARM being in the early stages. I've been running linux on a variety of ARM devices for ~8 years now, since the NSLU2 days. Good support for a huge variety of ARM SoCs has been in place for many years now.