After months of promises, Microsoft finally announced availability for the new Windows Phone 8 software development kit (SDK) at its Windows Phone event in San Francisco today. The SDK will be available for free starting tomorrow, and like older Windows Phone SDKs, it will be made available for download from Microsoft's site. More information will be available tomorrow at BUILD.

The new SDK will give developers access to all of Windows Phone 8's new APIs—Direct3D and some Win32 and WinRT APIs chief among them. It also includes a full Windows Phone emulator meant to help developers test their apps against devices with different screen sizes, much like the emulator included in the Android SDK.

With Windows Phone 8, one of Microsoft's stated goals has been to ease the porting of code from Windows 8—the two operating systems now use the same Windows NT kernel, and Windows Phones can now run apps coded in C++ and C in addition to the C# and XAML already supported in Windows Phone 7. New enterprise features, improved multitasking, and a speech API that's fully usable by third parties are among the platform's other under-the-hood enhancements.

This SDK has been available in a preview form since mid-September or so, but only to a select few developers who had already released apps for Windows Phone 7. The lack of lead time is new for developers, who often have SDK access for two or three months before a new platform launches—the most likely reason for the wait is that up until now the software just hasn't been finished.

Microsoft originally promised SDK access to developers by the end of the summer, but has missed that goal by quite a bit. Applications developed for Windows Phone 7.5 will continue to be compatible with Windows Phone 8, but they won't be able to take advantage of features in in the new operating system, and it may require quite a bit of effort for developers to port these apps over. The new APIs will also require quite a few changes, especially to 3D apps, and it will take some time for Windows Phone 7 developers to switch gears.