two devices one app?

Hi Guys,

As I am minutes away from receiving my Surface - I have a question that I have not found the answer to yet:

If I buy an app on my Windows 8 desktop (and assuming it works on RT) - do I have to repurchase the app like you would on iOS (iPhone vs iPad app)? Or does one purchase work with the account as a whole and I can download it on any device I am signed in on?

I think this is an important feature/question for Microsoft's push in the mobile game.

Thanks guys!