If you have a locked SIM card and you try to upgrade to the new iOS5 beta you will fail hard. I tried rolling back iTunes and the only thing that will work is the following.
- Pop out your SIM card
- Put it in another iPad
- Unlock it on the iPad under Settings > General
- Make sure you turn the setting to lock Sim Card to OFF
Pop it out and put it back in your device and you’re good to go. I really wish Apple would warn you about upgrading the iOS with a locked SIM card. Pretty scary and painful to fix.