From the installed NAnt folder copy the nant.xsd file to C:\Program Files\Microsoft Visual Studio 9.0\Xml\Schemas

In Visual Studio 2008, create a new XML file, add the root node <project>.
When you add the xmlns attribute to the project node the nant schema will now be available in the drop-down list.

NAnt intellisense and schema validation will now be available for the XML document.

Links:
NAnt website