Unity today announced the open beta program for Windows Phone Store apps. So, developers can start developing Unity 4 apps for Windows Phone devices.
If you enroll in both the Unity 4 Windows Store apps and Windows Phone 8 apps beta programs, you can develop games for the entire Windows ecosystem. People around the world can play your games on computers running Windows XP/7/8, Windows RT tablets and Windows Phone 8 mobiles.
Unity also noted that Unity 4 Windows Phone 8 support has not yet reached maturity and they are still working on performance optimizations. They will be fixing bugs and adding missing features until it will be finally released along with Unity 4.2. Read more about it from the link below.