I've got an iPhone 4S with an AT&T compatible Straight Talk SIM. It runs on AT&T's network and the iPhone acts just like it would if it had an AT&T SIM. The only was you can tell it's on ST is that the carrier field says "HOME". Talk and text work as soon as the SIM is inserted. Data requires a custom APN profile which you can get from http://unlock.co.nz MMS needs either a jailbreak or a non-AT&T SIM to access the APN settings on the phone. Search for "iPhone Tmobile MMS Sim Swap".
Straight Talk unlimited everything plan for $45