While Microsoft has remained silent on the subject of Windows Phone 8.1 on the record, the company is reportedly working to give at least some app developers access to the operating systems SDK ahead of its official reveal.
The Verge reports, via unnamed sources, that early SDK invites are being given to a select number of developers and partners, but it's not clear just how limited the access is at this point. Microsoft is supposed to officially announce plans for the next version of its mobile OS at its BUILD 2014 conference in San Francisco in early April, and is also supposed to release a developer preview version of Windows Phone 8.1 around that same time.
The Verge also claims that Microsoft will hold a secret meeting in London later this month for business customers to give them a technical preview on Windows Phone 8.1 and a road map for future updates.
Previous rumors about what will be included in Windows Phone 8.1 include the addition of a long awaited notification center. A slide that reportedly shows what that will look like was leaked on the Internet this past weekend. Other reports claim that a digital assistant, code named "Cortana", will make its debut with Windows Phone 8.1
Source: The Verge