Its pretty simplistic than you would think. I was searching my options to have intellisence in NHibernate mapping files so that i could edit them with convenience. All you have to do is to copy the schema files (.xsd) that has a definitions for intellisense and paste them in to the %ProgramFiles%\Microsoft Visual Studio 8\XML\Schemas folder. In the case of Hibernate, you have to copy the following files {nhibernate-configuration.xsd, nhibernate-generic.xsd, nhibernate-mapping.xsd} from the \src\NHibernate to %ProgramFiles%\Microsoft Visual Studio 8\XML\Schemas.
references :
http://blogs.msdn.com/astebner/archive/2005/12/07/501466.aspx
