Last Thursday Apple quietly issued a new build of iOS 5.0.1 specifically for the iPhone 4S. There were no release notes issued with the new build and users who had already installed and earlier build of 5.0.1 on their iPhone 4S couldn't update to the new one without restoring their iPhone. The release came one day before Apple rolled out the iPhone 4S in an additional 20 countries, which gave some hint that the build was useful for those markets.
Now the mystery has been solved as Apple released a new support document detailing changes in the new build. The build addresses an issue where the iPhone 4S wouldn't recognize a micro-SIM card that was installed and return a "No SIM Card Installed" or other similar error. It's also now apparent that the release of the build was meant to coincide with the 20 country rollout, as many new iPhone 4S owners would be upgrading from the factory installed iOS 5.0, to the new iOS 5.0.1 build when they bought their new iPhones.
The new build is still only available to people who have not previously updated to iOS 5.0.1, which suggests Apple considers the incidence of the "No SIM Card Installed" error fairly low. The fix for it will certainly be rolled into iOS 5.0.2 or iOS 5.1, whichever comes first. Until then, if you do want the absolute latest build on your iPhone 4S, simply restore the device to factory settings in iTunes.