If you're looking to save money with your GSM iPhone, Straight Talk provides a pre-paid option without unlocking or jailbreaking. Straight Talk offers iPhone owners an unlimited data, voice and text plan for just $45 per month. This is less than half the price of standard plans from AT&T.
Straight Talk is a MVNO (Mobile Virtual Network Operator), which means the carrier operates using other networks including AT&T and T-Mobile. Once you get a micro-SIM card from Straight Talk for your AT&T iPhone, the device won't know the difference.