One thing that's important about this announcement is that the SDK is non-commercial, free, and explicitly targeted at "academics and hobbyists," although I'm sure that Microsoft's going to be reaching out to professional game developers also. It also enables the hacker ethos by really letting developers get beyond the original gaming focus of the Kinect and get into the guts of its capabilities. From the Engadget article: "Built on XNA, the Kinect library is standalone, so you won't necessarily need to rely on DirectX being present. The SDK gives full access to everything the peripheral has to offer, including both cameras (VGA and depth-sensing) and the full microphone array. The former can identify up to six individuals or track the full skeletons for two, while the latter can handle advanced echo-cancellation and even sound triangulation."
Of course, people have been hacking the Kinect, and hooking it up to PCs since the Kinect was first released, and early on there were even rumblings that Microsoft was going to try to fight Kinect hackers, though they swiftly relented, then grew to embrace the movement. The release of this SDK is of course a culmination of their embrace of the movement, and an attempt to control and co-opt it.