With the release of VS 2008 and .NET Framework 3.5 we have installed our updated intellisense files to the EN folders for the files that had previously shipped, 2.0 and 3.0 ..those files override the files that were previously laid down by other installers. The intellisense logic looks first in the culture subdirectory EN before looking to the root folder.
For 3.5 we only needed to install the files to the root as those had never shipped before. I have directory structure of intellisense files for 2.0, 3.0 and 3.5 below:
NET 2.0
C:\Windows\Microsoft.NET\Framework\v2.0.50727\EN
.NET 3.0
C:\Program Files\Reference Assemblies\Microsoft\Framework\v3.0\EN
.NET 3.5
C:\Program Files\Reference Assemblies\Microsoft\Framework\v3.5
Hope this helps.
Anand..
http://blogs.msdn.com/sandcastle