Yesterday, we reported that Apple had released a tweaked version of iOS 5.0.1. The purpose of the tweak was unknown until just recently today. The new version arrived as Build 9A406, and apparently fixes the ‘No SIM Card’ error.
Here is a support page that recently surfaced on Apple’s website which seems to explain the update.
Apple recommends this solution for the SIM card issue:
1. Back up your user content and settings using iTunes or iCloud. See this article for more information.
2. Restore your iPhone 4S using iTunes.
3. After the process is complete, restore your user content and settings from your backup.
4. To confirm that you’ve successfully restored, verify that the version is now iOS 5.0.1 (9A406) in Settings > General > About.