iphone (1st gen, 3G) att sim card ?

Ya sure you can use AT&T sim in the new iphone freely only if it is unlocked one or else already locked with same AT&T network provider else if the iphone is locked to some other network provider then it wont accept your AT&T sim until you unlock it ..You can easily unlock the iphone through online providers like WickedUnlock.com here they are rendering remote unlocking service to unlock all model iphone from network provider ...You first check that whether the service is available for your country and network provider .If yes then pay for it and unlock the iphone then freely use it with any GSM sim ...