The WSS v3 & MOSS 2007 downloadable SDK files have been refreshed on Microsoft downloads site and were posted for download yesterday... refreshed on July 13, 2007. A note at the bottom of each downloads page tells you where the files are extracted on your system.
Note that the best and up-to-date documentation can be found online on MSDN (WSS v3 & MOSS 2007). This is also where you'll find the community contribution content. If you find something undocumented or something you think would be helpful to others, please post them to the community contribution on the appropriate page in the SDK!