As per our conversation in IRC, something else I'd like to highlight is the fact that in the current version of the spec any web site can host an app manifest for any web app. That means for example that I could create my own app store for web apps and provide a listing for a GMail app which users can add to their homescreen, without any involvement from Google.
It would be good to hear some opinions on whether it is a good thing or a bad thing that manifests don't have to be served from the same origin as the web app itself.
