The new iOS 5.0.1 build released yesterday may have been seeded to fix 'No SIM Card' errors on the iPhone 4S, according to MacRumors.
on a new Apple support document, it now appears that the updated build
may have been designed to address an issue some users have been
experiencing with SIM card-related errors. With the update affecting a
relatively small portion of users, Apple may have elected not to push it
out to all users in the form of an iOS 5.0.2 software update, and
instead rely on those users who are having problems following Apple's
advice and obtaining the new build through the restore process outlined
as a solution for the issue.
In the support document Apple
says that in certain situations, iPhone 4S may have difficulty
recognizing a micro-SIM card, resulting in the following behaviors:
● "Invalid SIM" or "No SIM Card installed" alert appears intermittently.
● Status bar displays "No Service" or "Searching" in a location with good network coverage.
resolve the issue, users are advised to restore their devices through
iTunes which would install the new build 9A406 of iOS 5.0.1.
There have also been early reports that the new build improves battery life for some users.
You can find download links for all the various iOS versions here.