Answer:
Nope, in the US, there is currently no way to buy a smart-phone (with a data plan) without a 2 year contract.
The first answer is wrong.YES,you can use a PrePaid Sim in a Samsung Captivate.Just pop your AT&T prepaid sim into the device and everything will work like it should BUT it is best to only use WiFi for data since Android phones go through a lot of it quickly and using 3G can drain your prepaid account quickly.If the phone is unlocked you can use other GSM carrier sims as well,such as T-Mobile but you would have to reconfigure the messaging and data settings for that service provider.