There are a few ways of getting Intellisense in your SharePoint XML Documents in Visual Studio:
Feature and Elements Manifest XML files
Site Definition Site Template XML files
Add the following SharePointCatalog.xml file to C:\Program Files\Microsoft Visual Studio 8\Xml\Schemas\ folder.
<schemacatalog xmlns="http:>
<Schema
href="C:/Program Files/Common Files/Microsoft Shared/web server extensions/12/TEMPLATE/XML/wss.xsd"
targetNamespace="http: />
</schemacatalog>
Ensure you include the namespace in the Site Definition Site Template XML file:
<?xml version="1.0" encoding="utf-8"?>
<Project xmlns="http:>
</Project>
Portal Web Manifest XML files
TBA (anyone?)
Below is example code that out of the box you will not intellisense on (even with settings configured above).
<?xml version="1.0" encoding="utf-8"?>
<portal xmlns="PortalTemplate.xsd">
<web name="Home" siteDefinition="CMSPUBLISHING" displayName="Home" description="Home">
<webs>
<web name="Services" siteDefinition="CMSPUBLISHING" displayName="Services" description="Services" />
</webs>
</web>
</portal>