All, 

The newest versions of the SharePoint SDKs are published on MSDN. For the best links on current documentation, including the impact of federation, infrastructure updates and much more in the SDKs, check out these sites:

http://blogs.msdn.com/sharepoint/

If you want to go straight to the downloads and get the goods:

MOSS - http://www.microsoft.com/downloads/details.aspx?familyid=6D94E307-67D9-41AC-B2D6-0074D6286FA9&displaylang=en

WSS - http://www.microsoft.com/downloads/details.aspx?familyid=05E0DD12-8394-402B-8936-A07FE8AFAFFD&displaylang=en

Best,

Scott