This other article also comes from one of my answers in the MSDN VSX forum:

HOWTO: Force files to open with a specific editor from a Visual Studio package

It explains how to overcome a limitation of the automation model (EnvDTE) forcing a ProjectItem to open in a specific editor even when the user has set the default to other editor (especially problematic if the editor is external, such as Notepad). It is also useful if you want to start getting rid of “EnvDTE” code in your package.