1. Turn on the phone.
2. Enter sim pin.
3. Perform a hard reset (hold down power and home buttons until phone turns off, and keep holding until Apple logo appears).
4. When the phone comes back on, slide unlock. It will not ask for a pin and will work. You will see searching after 5 seconds, and after about 30-60 seconds, the network will come online.
Now if someone came up with an app that performs a hard reset(not reboot...).