For a number of security reasons, this isn't currently possible.
However, handling mobile phone logins gracefully is a great idea, and
I'll be sure to let the Friend Connect developers know there's
interest.
-Bob Aman
Actually, I just went through the login process on the iPhone, and
honestly, it's not that bad. The popup window could certainly be
reformatted a bit to be more readable for that screen size, but it's
totally usable the way it is, at least when signing in with a Google
account. It should be noted though, that if any OpenID providers
don't reformat their sign-in process for mobile devices, you will
still see the large window size problem when logging in with them, no
matter what Friend Connect does.
-Bob Aman