2006 Saturn Ion, automatic transmission. When I turn off the ignition the engine will die but the key will not turn past the "ACC" position. Sometimes I can turn the wheel left and right, return it to the center position and it will allow me to turn it to the "OFF" position. Sometimes I have to turn the key back to the "ON" position, shift from park to reverse and back to park and it will turn to the "OFF" position. Sometimes pushing the gear shift button repeatedly will allow it to turn to the "OFF" position.
Most likely you have a bad micro switch that is at the bottom of your shifter shaft that releases the solenoid that allow you to turn your key off in park. Pretty common on that car also I have seen some shifters stick and cause the switch to not release the solenoid.