My iPhone 4S running iOS 5.0.1 recently started showing "invalid sim" message
This is a very common problem which has caused problems for a huge number of iPhone users, particularly outside traditional markets.

There is every reason to believe that it will be fixed by the imminent 5.1 iOS release.

It appears to have something to do with carrier settings, GSM frequencies, and/or SIM cards.

There is some detail in this thread ...
https://discussions.apple.com/message/17769511?ac_cid=142432#17769511

If it is not fixed in the 5.1 release, there will be a lot of very angry iPhone 4S users!