Great news have just come to my attention: after months of hard work by the Unity3D team, they have finally released awesome news for all of you working with Unity3D - you can now start porting your existing games or creating new games for the Windows Store using Unity3d.
As the team blogged quite recently, they decided to open up the beta enrolment program to people interested and enthusiastic about the platform. However, you should be somewhat experienced with it and also you should not release anything time critical using it just now. It is still in beta, after all, and that means that a bug or two might pop up, stability isn't guaranteed and some of the final features are still missing, given that the team is still working on it.
However, you can help to make the platform even better! By using it and reporting your feedback, you can help the team with their final stretch of work, so if you feel you are experienced enough and want to play around with it a bit, then go on to this enrolment site: http://unity3d.com/beta/windowsstoreapps
No, seriously, what are you waiting for? :) Enrol now ;)
PS: be aware, this is only for Windows Store games, i.e. Windows 8 / Windows RT only.
 - http://blogs.unity3d.com/2013/03/07/unity-4-early-access-for-windows-store-apps/