iPhone 3GS always rebooting when logging on to cellular network I upgraded to 4.3, without jailbreak. Phone seemed ok, but I turned it off and on again a few days later (normal power off). It started rebooting itself. I tried hard resets. 

Eventually got to a stage where it needed the SIM unlock code but was otherwise working. Unlocking the SIM froze the device when the signal bars first showed (trying to register on the network, I guess). I tried resetting the network settings, no help.

I did a complete restore this morning, setup as new device, activated (officially unlocked). Still the same problem. Someone at work connected it to XCode on Mac, said they didn't really do anything, and it started working. I turned it off, on again, and it has the same problem. Any ideas?
You might be lucky, iOS 4.3.1 is out and there's the following change in the changelog:


  Resolves bugs relating to activating and connecting to some cellular networks


Try to update and tell us if it fix the problem.