Windows Phone 8 SDK has been released as documented by Next Web:
QUOTE: Yesterday Microsoft promised to release the Windows Phone 8 SDK at its event in San Francisco. Today, during the opening keynote of its BUILD conference, the SDK went live. Microsoft is betting heavily on Windows Phone 8, moving the core of the operating system to a shared codebase with Windows 8. While this presents numerous advantages, it does cut loose all older Windows Phone handets, as their physical architecture cannot manage the new software.
Available from link below: