firstly, you need to purchase an unlock code for example from horizon wireless.
secondly, no. if you plan on using a pre-paid sim card (thats why you need to unlock the device) you would only get calls/texts, as 1) almost no carriers offer BIS data plan with pre-paid and 2) it wouldn't registered with the BIS because your currently using it on your current account.
"That's it then. Cancel the kitchen scraps for lepers and orphans, no more merciful beheadings... and call off Christmas."
AT&T will, within certain restrictions, provide the unlock code for you free of charge. You must be a customer in good standing (no past due balances), must have been a customer for 90 days, and must have a valid reason for requesting the unlock code (traveling overseas IS a valid reason in the eyes of AT&T).