Question regarding "METRO" apps on windows phone 8 (ARM) / windows RT (ARM) / windows 8 (x86)

What is compatibility going to be like?

The metro interface is a constant now between all devices in terms of basic layout, but the underpinnings of devices like laptops are very different from ARM chip phones and RT tablets, so if you buy an app in the windows store will it be able to run on every windows 8 device?

If not how much work will it take make an X86 compatible metro app work on an ARM device like windows RT and windows phone 8?

I ask this because depending on the ease of transition from metro apps on computers to tablets and phones, it's kind of game over in terms of the apps race.

Most new windows pcs sold will be loaded with windows 8, complete with the window store that sells metro apps, the user base for those applications will be massive by default because of the number of people out there in the world that use non apple computers.

That seems like a clear path to a placing in the mobile phone and tablet and computing market, am I missing something?