What is the difference between iOS 5.0.1 build 9A405 and 9A406?
iOS 5.0.1 was released by Apple in two versions, build 9A405 and 9A406. The second build was released to help correct a problem affecting iPhone 4S owners receiving a "No SIM Card Installed" error on their devices. The update was not released as iOS 5.0.2 to all users since the fix was specific to the iPhone 4S and was only intended to help a small number of users affected by the problem.
If you haven't already updated to iOS 5.0.1, the new build will automatically be the version you receive on your iPhone during the update. Those who have already updated to iOS 5.0.1 will not be notified of the new build. To install 9A406 on the iPhone 4S users can follow the instructions in this Apple Support Document.