OK, so I just was using my iPhone 3G and I looked in the upper left hand corner and it had the "No Service" message in place of carrier name etc. Now it had service less than half an hour ago, as I talked to a friend, and I've checked two other cell phones and they both have service. What happened? I've tried turning Airplane mode off and on several times. I even restarted the phone, but nothing. Is this a serious problem? Thanks.