The SDKs for Windows SharePoint Services 3.0 and Office SharePoint Server 2007 has been updated and are now available at the Microsoft Download site.
Windows SharePoint Services 3.0: Software Development Kit (SDK) (version 1.3)
SharePoint Server 2007 SDK: Software Development Kit (version 1.3)This contains several updates and worth to notice is the Search sample protocol handler, Custom Content Source and the BDC editor.