I had to do some development to move an old addin over from Office 2003 to Office 2007 and that is why I needed to edit the Ribbon XML in VSTO 2005 SE (don't forget to update). Well, doing that without the Intellisense support is a tough one to call. So a reminder to self because I keep forgetting how to get Ribbon XML Intellisense working in VSTO 2005 SE with a fresh install:

Copy the file

\program files\microsoft visual studio 8\Xml\Schemas\1033\CustomUI.xsd

to the folder

\program files\microsoft visual studio 8\Xml\Schemas\

That will give the Ribbon Support Intellisense in VSTO 2005 SE and making your job a lot easier to do!