The identity indeed doesn't have to be an email address; I'm currently using a random thing I set up on AppEngine's free tier (serving a static file for the public side; the auth side is client-local). There's no way I'll be receiving @*.appspot.com mail. Of course, this also means my identity is tied to a domain I don't control and might go away at any point with no warning...

I do think that the Persona developers are conflating identity with e-mail too much; I can't use it for Mozilla's bug tracker, for example, because I actually value receiving mail and they have no plans to have split identity/e-mail information.