Usually when you use a gophone sim that came with free minutes because of a promotion bundle (like buy a phone and get $25 free minutes type of deal) they will lock the sim to the phone for 180 days. There are other options besides AT&T prepaid if you don't need a data plan and just want a working iphone for cheap.
If you want to use AT&T prepaid and want to make sure you get a SIM that will activate your phone you can buy the cheapest candy bar type Nokia phone like the 2330 from the AT&T store for 7 or 8 bucks shipped. They will come with a Sim you can use. You can set it up and get it all working in the Nokia phone then just swap it into your iphone.
These are the other providers that I know of that you can use their sim cards in an iphone without a data plan and without being jailbroken and unlocked because they use the AT&T network:
Pure Prepaid
Pure TalkUSA
7-11 Speak Out wireless (be careful some use sprint and some AT&T)
Consumer Cellular
GTC wireless
Jolt Wireless
Locus Mobile (be careful some use Verizon and some AT&T)
TracFone (be careful they use multiple providers including AT&T)
Personally, on the phones I do not want data plans on I use Pure TalkUSA. You can by a Sim only with minutes that rollover for $10 and as little as $10 a month to keep it going.
