I have had my Iphone 4 for a couple of months now and I have probably gotten the "No SIM card inserted" error message at least 15 times. I am using the SIM card that came with the phone. I carry my phone in my front pocket so the only "jarring" it gets is from normal walking. Turning the phone off and then back on fixes the problem. Still, its annoying to have to reboot the phone plus I may miss phone call while the phone is in my pocket and I am unaware that the SIM card stopped working. Curious to know if anyone else has encountered this problem.