How do I unblock my SIM card?

I recently received my new AT&T phone (they're taking over Alltel). I was playing around with my phone yesterday and accidentally locked my SIM card. It is now asking me for a PUK code, which I could get if I logged into my AT&T account... Only problem is, since Alltel hasn't switched over to AT&T yet, I don't have an account being as my number is not in their system. Is there another way to receive a PUK code or am I stuck until my phone and account are activated?