As I think about providing an API for apps to easily integrate* *signing in
with Persona, I also wonder at the possibility of an app being an IdP
For example, most people would have a Twitter, Gmail, Yahoo Mail, or other
app that might be of the place they host their identity. But they likely
aren't logged into those services in the stock browser. It would be a
better experience if the app could provide the authentication, instead of
the user needing to login through the browser.
A native app could try to register that is is able to provide a login for
the user. I know in Android, it would simply turn the request into a
standard action picker "Login using one of these apps", it would include
the Browser, among others that register the intent.