[derefr]

There's a sleight-of-hand Amazon plays with their own sign-in box: they give you a single "email" box, and then two radio buttons -- "I'm new" and "I already have an account and here is my password" [with a password input below that option].

The clever thing is, the radio buttons are completely ignored -- if you have an account and the password matches, you get logged in; if you didn't put in a password, and the email isn't in their records, they bring you to the account creation flow. The radio buttons are just there to let users express a choice they expected to be able to make, and thereby keep them in flow.

A better Persona login box could just do the same thing, but without the password input box under the "I already have an account" option. In fact, since selecting an option is the last step of the flow, just have an email field with two buttons, "Sign Up" and "Log In". Both buttons do the same thing :)