The folks over at TheCodeproject have a rudimentary GUI for sandcastle --> http://www.codeproject.com/lounge.asp?msg=1602333#xx1602333xx
I decided to take on the task of writing a MSBuild script for Sandcastle. Have a look at it if you're interested!
http://blog.ljusberg.com/2006/07/msbuild-script-for-sandcastle.html
/Anders