While this certainly doesn't appear to be as widespread a problem as iOS 5's battery issues, a small group of iPhone 4S users are reporting that that their devices randomly fail to recognize the installed SIM card. This pops up an error message and, more importantly, prevents users from making phone calls or using cellular data — basically, anything requiring a signal. These reports started coming in to Apple's support forums from users across a variety of countries and carriers shortly after the iPhone 4S launched, and they've persisted over the last month. Even CDMA iPhones on Verizon and Sprint are having issues, despite the SIM card being inactive.
There doesn't appear to be any official remedy at the moment — users have reported that rebooting helps, but the problems eventually return and in some cases stop responding to the reboot. Others have reportedly gotten new SIMs from their carrier, or just gone straight to Apple to exchange their phones. There's been no official word from Apple nor any response to those with trouble on the support forum, so don't hesitate to make Apple replace your phone or SIM if you're having trouble.