Some iPhone 4S owners have been experiencing a peculiar situation where the device stops recognizing their SIM cards. Users have been reporting that they receive error messages on the iPhone 4S stating that they either have no SIM card installed or their SIM is invalid. Rebooting the handset works as a temporary solution, but most users experiencing this problem have reported that eventually the iPhone 4S will refuse to recognize the SIM at all.
This issue doesn't appear to be isolated to any one carrier or model of iPhone 4S, and it's been happening to quite a few users since the handset launched. So far, the only sure-fire solution to the issue seems to be getting a replacement SIM from your carrier and swapping out the old one.
It's not clear if this is a hardware issue specific to the iPhone 4S, a software bug in iOS, a bad batch of SIMs across multiple carriers, or some unlikely combination of the three. Users have reported success in getting their iPhone 4S units to recognize new SIMs, so if you experience this issue, a visit to your carrier's closest store is probably in order.