There is a unlock service only charge $5.99 now. However, I do not try it so I do not know whether it works or not.
http://www.iunlock247.com/usa-carriers/unlock-your-iphone-2g3g3gs44s5-from-att-united-states/
http://www.iunlock247.com/usa-carriers/unlock-your-iphone-2g3g3gs44s5-from-att-united-states/