input - 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?

Answer

First, keep a non-exiperable cookie with email so you could always identify if it is a returning user, even if his login session has expired and he has no autofill on.

Second, when user completes email field, send an ajax request to determine if such a user exist or not. Depending on that, choose tab. Fields are the same, so do not clear them when switching.

Third, you will need to allow users to choose what they want to do, create account or sign in, even if they misremember or misspell their email. If the user choosed tab by hand, then he knows what he wants. Do not auto-switch tabs after that, instead, show messages like 'no such user registered, create an account?' or 'user with this email already exists, sign in?'

Single button as @Asmor propose won't work, because user will be no clear way to choose desired behavior (I want to login, but button states "Login or create", how do I login?) and also he will not be able to tell what will happen after he clicks this button ("will I be logged in or new account will be created?").