Those who wanted a local copy of AutoCAD Civil 3D 2012 API Reference docs, I have a good news to share with you
Civil 3D 2012 API Reference documentation is now available for download from Civil 3D Developer Center Page http://usa.autodesk.com/adsk/servlet/index?siteID=
AutoCAD Civil 3D .NET API reference document is another very useful reference document with code snippets which covers the Civil 3D .NET objects. AutoCAD Civil 3D 2012 .NET API reference document is now available for download.
Note - This is a one-time only post; any updates and documentation fixes made in the future may not be distributed, and the latest documentation can only be found online.