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.

If you are into this sort of thing, you can download the SDK here.

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.

Windows Phone now sports over 120,000 applications, but the ‘density’ of those apps, which is to say their granular and marginal quality, has always been in doubt. However, the company did announce yesterday that some 46 of the top 50 apps would be on Windows Phone shortly.

wp8 crop The Windows Phone 8 SDK is now live for all, heres where to get it

That Microsoft has released the SDK today is no surprise. BUILD is a developer event, packed to the gills with folks who write code for Windows, whom Microsoft would love to make skip, jump and hop over to building for Windows 8.

TNW will have more from BUILD as we sit in on the most key developer sessions.