Microsoft's documentation generator, codenamed Sandcastle, is available for download now.  Keep in mind this is a very rough CTP.  It's all command-line driven, though some community efforts have made scripts, etc. to simplify things.  I've run it, and was able to generate a .chm, though with much more effort than nDoc. 

The lack of UI brings up an important point.  This was the code that Microsoft was using to generate MSDN developer content.  Most likely they used it as part of a continuous integration process that generated new documentation in the background.  While GUI may make things easier up front (and I definitely want one, Microsoft), thinking about integrating documentation seamlessly into our build processes using MSBuild and scripts may be helpful as well.