If the phone is either unlocked or locked to ATT you won't have a problem. You can visit howardforums.com for additional info on configuration if necessary.
I was told by my AT&T provider that I cannot use a Blackjack II without buying a contract. Couldn't I just buy one of of a web site w/o a contract and slide in my SIM card and it work? I wouldn't think it would take any unlocking to do so.