I wanted to enable intellisense in visual studio with NAnt and i saw this great blog article on Scott Hanselman's blog. However, it was a little bit out of date for my taste.... so what i did was this:

I went and did a search on google for "Nant XSD"

then i did a save target as... on this link

and i saved it to:

C:\Program Files\Microsoft Visual Studio 9.0\Common7\Packages\schemas\xml\nant.xsd

Then inside visual studio... i opened my *.Build file and went to the properties pane. Where it said "Schema" i clicked the "..." and added my new nant schema into that list and 'selected' it.