Or just buy it full price and save the bother.
Full price 16gb iPhone 4s is $649 plus tax. Out the door it is unlocked and you never have to deal with AT&T for the unlock, for fees etc.
Subsidized iPhone is $199 plus tax (which in about five states is on the full price) plus $36 activation fee and a trip to AT&T to pay the $450 eft etc. and they might make you pay that first months charge on top of the non refundable activation fee.
So in Cali that's $705 and I'm ready to go from Apple to t-mobile or at least $741 and I have to deal with AT&T who I am trying to get away from.
Even outside of those five states the math doesn't add up. In Memphis where I just finished a job and bought my iPhone the sales tax is 9.25% for that 16gb phone. So full price that's $709 and I avoid AT&T or at least $703 and I have to talk to them.