I just read code P0128 from my grand prix GT too. It was 7 degrees that morning.
Can you believe the dealer wanted $80 to read the codes from my car? so I said screw that and went and bought an ODBii interface from scantool.net. It works great and now I will be checking my own codes and making my own repairs.
I'll clear this code, and if it comes back I'll probably try replacing the thermostat.
Good to see other people have the came car and are experiencing similar issues. At least this way I'll know what to expect on my car.