I have the same problem. I'm not in the US and my iPad I is not locked. It was working great till upgrading to 4.3.3
I have tried replacng the sim with another operator's sim. Even tried it in a different country with a data sim that was working on other devices.
Sometimes a signal would appear but soon the corner shows " searching " then " No Service"
Multiple searches I did showed some complains in US, Australia and other countries.
Turning Airplane off and on again, rebooting, reinstalling the system, restoring settings, hard reset with power off for more than 6 hours as one of the advices said but no luck.