Hi Stephane,
As Rob has said, my goal was to present the "full setup" In Gateway/Router mode.
This way the user can test all the options of a real NS installation and not only server mode.
Plus, now it will be simple to add more VBox clients that are connected to vboxnet0 and this way you create your own virtual lan with machines behind NS that will provide them with access to internet. (you can now do what you do with VmWare virtual switches 
).
I hope this will help some new user.
it will also be linked with the Dev how-to because it was one of the requirements to be able to play with NS in a virtual environment
PS: If anyone has followed this How-to please give us your feedback