Earlier today, Buck Hodges blogged about IntelliSense while editing Work Item Types. In his post, he noted that the schema for work item types (as well as the other schemas used by Team Foundation Server process templates) is located in the Visual Studio 2005 SDK.
However, if you don't want the entire SDK, you can simply download the process template schemas from this help topic in the Team Foundation Server product docs: Process Template Schemas Download.
Run the download (ProcessTemplateSchemas.EXE), and when prompted, extract the contents to:
%ProgramFiles%\Microsoft Visual Studio 8\Xml\Schemas
Of course, you'll need to specify the actual value for %ProgramFiles%, such as:
C:\Program Files\Microsoft Visual Studio 8\Xml\Schemas