Quick update : I drove down to the US border last night to try the GoPhone SIM card in my iPhone 4 within range of an AT&T tower.
Turns out I'm getting the same behavior as I did at home in Montreal : I either get the "SIM failure" message, or if I put the phone in airplane mode and reboot, I then get one network bar and no connection to the AT&T network.
After further research this morning, I have found that the baseband used by my iPhone, 04.10.01, can not be unlocked by ultrasnow. I'm at version 4.3.3 of iOS by the way.
I'm looking right now at other avenues, namely :
- trying to install a custom iOS (with different baseband) onto my phone, I don't know if that's at all possible
- trying the SIM in my old iPhone 3G (I don't remember which iOS version / baseband it was on).