There's an interesting point made, but I'm not sure that I really like the direction Leah went in. To me, the fields stand out far more than the radio buttons, I stop reading the header after "Log in..." (what more do I need to know, it's a login form) and I really wouldn't notice that it's a sign-up form unless I look carefully - I had to look twice before I realized there was even a difference between the two screenshots.

I'd probably ditch the radio buttons and make the header more eyecatching, with "Join Now?" clearly a link. Clicking it changes the form in the way Leah shows, probably with a quick Javascript fade so the user notices.