You can still use the prepaid phone sim with any unlocked phone with compatible 3G band. Go to Target/Best Buy/Walmart or similar store and buy the cheapest ATT GoPhone they sell ($10) + whatever amount of minutes you want to start with. Instead of using the cheap GoPhone, put the GoPhone SIM in your regular unlocked phone and activate it on the prepaid plan you want. Works fine.
My sister did this for her unlocked Nokia when she visited the US earlier in the year.