The initial load of the necessary resources still requires a connection; it's not like offline Gmail just comes preinstalled with a cache of your messages...
The point, though, is that a network connection is required to get the initial files and data to use for offline work. If you don't have that capability to do the initial load, you still can't demo an offline web app.
Just like you can't demo an iPhone app without having an internet connection to download it first! However, the iPhone, Windows CE, and Chrome OS ship with a number of apps preinstalled. :)
