If you purchase a discounted phone with a two-year contract and then cancel after the 14-day return period, AT&T will charge a pro-rated Early Termination Fee based on your phone/device type.
For Smartphones, such as the Motorola ATRIX and Samsung Focus: AT&T charges $325 minus $10 for each full month of your Service Commitment that you complete.
For Standard Phones, such as the Sony Ericsson W518a or Pantech Breeze II: AT&T charges $150 minus $4 for each full month of your Service Commitment that you complete.
For a complete list of the devices and their applicable fee, check www.att.com/equipmentETF
NOTE: Once you fulfill your service contract, you'll automatically become a month-to-month customer.