Jay Flowers sent me an email this morning, to remind me that you can recreate the NAnt XSD(Schema) at any time by running the 'nantschema' task.....Jay Flowers reminded me of nant's very clever built in 'nantschema' task which will create an xsd (named however you like) containing the schema for the entire suite of NAnt tasks present your copy.
This means that if you have downloaded NantContrib or have created your own tasks, then the 'nantschema' task will reflect over these and include them in the xsd it creates.
As Jay says you can have visual studio give you this information back as intellisense by setting the default namespace of your build file to the generated xsd.
Jay also gave a link to a good 'HowTo' which covers pretty much all you need to know
Thanks for the mail Jay