Earlier this month, Microsoft outlined the concept of universal apps which can run on both Windows 8.1 and Windows Phone 8.1 developers. From today, Microsoft is accepting submissions for universal apps.
As part of that process, Microsoft has made Windows Phone 8.1 available to registered developers. To build universal apps, you'll also need to install Visual Studio Update 2 RC.
Given that universal apps will only run on Windows Phone 8.1 and that hasn't gone into general release yet, there's probably no mad rush to do this, but it could be a tempting route for Windows Phone developers seeking to expand into potentially more lucrative desktop apps. Hit the link below for more.
Dev Center now open for Windows Phone 8.1 and universal Windows app submissions [Building Apps For Windows]