Apple released a new build of iOS 5.0.1 a few days ago, but no one knew what differentiated it from the previous build until now. It was actually issued to fix SIM card errors that certain iPhone 4S owners have been experiencing, according to one Apple support page. Specifically, the phone would have trouble recognizing the inserted SIM card.
Apple says the two main behaviors have been fixed with the new build. An alert would appear intermittently with a message reading either “Invalid SIM” or “No SIM card installed.” In addition, the status bar would display either “Searching” or “No Service” even if the phone was in an area with good coverage.
Since the issues only affected a small number of iPhone 4S users, Apple decided to send out a patch to iOS 5.0.1, rather than releasing what would be iOS 5.0.2 for everyone. The new build number is 9A406, which is only slightly modified from the previous build 9A405. To upgrade your problematic iPhone 4S device, be sure to back up the device with iTunes (I’m assuming iCloud backup would work too) and then restore from iTunes, after which you would then see build 9A406 in Settings > General > About.
The original version of iOS 5.0.1 was released in November for all iOS 5 users to fix abnormally short battery life in some devices, though apparently certain users still have battery life problems.