My iPhone 4S running iOS 5.0.1 recently started showing "invalid sim" message I recently forgot to pay for my cellular service, and when I paid the service my iPhone keeps displaying an "Invalid sim" message, then I have to reboot to recover the service.

The phone is working correctly with internet access, calls and messages, the only problem is that I need to reboot the phone two or three times a day to eliminate the "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!