Windows Driver Foundation (WDF) is a set of Microsoft tools used to develop drivers for Windows. It is available in two flavors: the Kernel Mode Driver Framework (KMDF) and the User-Mode Driver Framework (UMDF). WDF 1.7 is the latest version of the tools and it supports the largest number of operating systems yet: KMDF 1.7 supports Windows 2000 SP4, Windows XP SP2, Windows Server 2003 SP1, Windows Vista SP1, and Windows Server 2008, while UMDF 1.7 supports the same list with the exception of Windows 2000 SP4.
WDF 1.7 was working fine for many users until KB 938371 was released, at which point KMDF 1.7 would no longer install on Windows Vista. The update was a prerequisite update for Vista SP1, so naturally it took priority. The problem was supposed to be fixed by March 15, but then another issue was discovered with the UMDF coinstaller. The expected date for both versions of WDF 1.7 is now April 15, 2008, but even that is not currently guaranteed.
If you're interested in learning more about version 1.7, check out the detailed list of changes for RC1 (the final release notes are not yet available) that developer Ilias Tsigkogiannis posted last December on his blog.