Can I use a smartphone with AT&T prepaid?

If the smart phone phone is unlocked, It would definitely work with any GSM SIM card. But make sure how safe was the phone unlocked. Certain unsafe unlocking methods like flashing would give problems while using with other SIM cards. So it is safe to buy a network locked mobile phone and unlock it yourself safer and easier using unlock code. You can get the accurate unlock code for a mobile phone from Unlocking4U.com at affordable cost with an easy unlocking guide.