Just to clarify: the SIM you had in your Australian phone is an AT&T SIM? If it's an AT&T SIM that has been active in the last three months, you should be able to simply refill it.
Do you happen to know your account number, since you've forgotten your phone number? When you were here in October, did you refill your prepaid account online? If so, you should be able to log in online and find out your number.
It may just be easier to set up a new account and pick up a SIM from any AT&T corporate store. Let me know if this helps!
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.