The Windows Phone 8 Developer Platform has been made available today. It includes a new SDK and a refreshed Dev Center, open for Windows Phone 8 app submissions.

Microsoft made the SDK, along with a free version of Visual Studio Express 2012 and Blend 5, available to MSDN subscribers and to the public for download on October 30.

The announcement was made today at the //BUILD 2012 conference. Microsoft's Windows Phone 8 is delivered in close alignment with Windows 8 via a common core. Moving to a common Windows core meant that almost every major underlying subsystem had to change. For example, for the .NET Framework, Microsoft moved from using .NET CF to Core CLR, two different versions of the Framework that forked from each other over five years ago. This gives developers far more capability.

This allows developers to take advantage of support for native C++ programming, familiar tools, and common APIs to target phones, PCs, and tablets for an estimated combined opportunity of roughly 500 million units next year.

Hopefuly, Windows Phone 8 can run Windows Phone 7.X apps. But unless developers were among the chosen few who got early access to the SDK, they haven't been able to build apps that take advantage of the new Windows Phone 8 features, like NFC, Wallet, and multicore until today.

Microsoft says that industry and customer response has been fantastic, and Windows Phone will be available through 50 of the top mobile operators worldwide starting next month.

Steve Ballmer announced that since Windows 8 became generally available on October 26, Microsoft has sold four million upgrade copies of the operating system.

Ballmer and other Microsoft execs also announced coming availability of more Windows 8 apps, including a new Twitter client being developed by Twitter itself. Other developers who have committed to new Windows 8 apps, as announced today, include SAP, Dropbox and ESPN.

Microsoft's Dev Center offers guidance on how to get started with the new SDK.