Got this from Scott Guthrie's blog:


Now one annoying gotcha:

 

There is one gotcha to be aware of, though, that can sometimes cause intellisense for the web.config file to stop working in the IDE.  This happens when a default namespace is added to the root <configuration> element.  For example, like so:

 

            <configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">

           

This doesn’t cause any runtime problems – but it does stop intellisense completion happening for the built-in .NET XML elements in the web.config file. 

 

The bad news is that the built-in web admin tool (launched via the WebSite->ASP.NET Configuration menu item in VS 2005 and Visual Web Developer) always adds this xmlns namespace when it launches – so if you use this tool to manage users/roles you’ll end up having it added to your web.config file for you.

 

How to fix this gotcha:

 

To get intellisense back when you are editing the web.config file in the IDE, just delete the xmlns reference and have the root configuration element look like so:

 

<configuration>

 

Everything will then work fine again.

Ref Link:
http://weblogs.asp.net/scottgu/archive/2005/12/02/432077.aspx
http://www.radsoftware.com.au/articles/intellisensewebconfig.aspx