I live in Canada, use Rogers, purchased (own outright) a Captivate, and called Att twice yesterday.
Both CSR's were helpful and tried to get the unlock code from their 'system' by using my IEMI, both told me that the lookup was not found.
I also tried unlockallcellular
Samsung i897 Captivate Unlock CODE + FREE Instructions Guide: Online Unlocking of Your Orange - FR Samsung i897 Captivate
and cellunlocker
Cellunlocker.net - The #1 Phone Unlocking Website. Unlock your phone by simply entering a code Fast and Easy
Neither site was able to unlock my phone. Unlockall has already refunded my cash... still waitign on cellunlocker.
It seems that the unlock codes are a tightly guarded secret. Obviously ATT and Samsung are able to unlock the phone, but I'm guessing the exclusivity agreement between the two is preventing the release of the sim lock codes to the csr's.
It's only a matter of time.. 8 digit simlock code.. 16 digit imei + carrier... They'll work out the hash, but if they don't by Friday, this phone is going back...