The latest version of the WDK, this one supporting operating system platforms through Windows Server 2008, has been released. This release is concurrent with the release of Windows Server 2008 to manufacturing.
You can download the WDK free by joining Microsoft Connect. You'll find the instructions here. It'll also show up (eventually) in your MSDN subscripion.
Remember, our prayers have been (at least partially) answered and the WDK folks have once again separated the useful, technical, content (the WDK, including the docs, build environment, headers, and libs) from the goofy, annoying, testing-oriented content (the WLK). So, you won't have to download 2GB of SQL-related animal waste in order to be able to build drivers with the latest version of the kit. The WDK download is a "lean" 635MB.
This new kit includes the usual updates to the build environments and documentation (but you've been updating your docs all along, haven't you??) as well as V1.7 of both KMDF and UMDF. This new version of the Frameworks includes numerous bug fixes as well as a number of new samples. Probably the most important thing for UMDF is that V1.7 now will install and work on Windows Server 2003 SP1.
Now that the Server 2008 WDK has been released, word has it that the WDK gnomes are already working on the next version, scheduled for release with Windows 7. I hear rumours there's at least one feature in the Win7 WDK release that's been long requested by the community. But, what it is I'm not saying. At least not yet.