[innocenat]

It is already normally possible to test whether email address is registered by trying to register with that email address. Unless that process is secured too, it doesn't really make much sense to not pop up Unknown email address error.

[pwman]

Correct -- It's a pet peeve of mine when login processes obscure this saying invalid password when the sign up process doesn't -- if you're going to tell people usernames aren't available then you shouldn't be avoiding it on the login screen.