In a welcome move, Microsoft has made it possible to post apps to both Windows store and Windows Phone store in one registration, which may be the logical first step towards unification of Windows RT and Windows Phone OSes going forward. So, developers don’t need to register twice and pay fees twice and as Microsoft has put it in their blog,
As a result of these changes, Windows Store developers are now also Windows Phone developers, and Windows Phone developers are also Windows Store developers. Welcome all!
The results of this move could be really favorable for the Windows app eco-system. Developers may be much more likely to bring their apps to both stores and as one can envisage, with WP 8.1 in 2014, he app development tools will have much in common for both OSes.