Best way to combine sign in and sign up in the same form?

I want both forms to be extremely simple - sign in much like facebook in the header with 3 inputs side by side - email, password then "sign in" (or is "log in" better?).

I also want to USE THE SAME FIELDS for a SIGNUP form, but I am stuck at the sign in button. I do not want two buttons, one for sign up and one for sign in, because that will confuse people. I just want to be efficient by using the same email and password fields. I dont need a password confirmation field. Any suggestions on how I can get this to work?