This page has a great resource for writing xml documentation for code in Visual Studio and Sandcastle. It outlines the differences between the xml support of Visual Studio and Sandcastle. It is a great resource especially because Visual Studio doesn't understand all the xml elements supported by Sandcastle. The document is now a year and a half old, but is still very accurate as the Visual Studio xml documentation schema and Sandcastle support haven't changed much (if at all) since.
Tags: Sandcastle