Ok, break out your favorite mobile reg editor and locate:
In here will be a value named "ForceCellConnection"
Change "ATT ICS" to "" and reboot.
ICS will now allow you to choose either ATT ICS or MEdia. ATT ICS will not work without a tethering plan on your wireless account, but MEdia Net will work as long as you have the signal and the basic pay-as-you-go feature (not recommended).
ATT ICS uses "isp.cingular" (tethering) whereas MEdia Net uses "wap.cingular" (general internet).
Also, the ATT ICS and MEdia Net entries in the connection settings dialog are read only and won't let you edit them, But if you navigate to:
There will be 2 keys in there. one for ATT ICS and MEdia Net. In those keys there will be a value of ReadOnly. Change the value for these from 1 to 0 and reboot your phone. These connections should now be editable.
This post was made via tether w/o plan and via USB connection