Summary of Post - Fixed the "Invalid SIM" / "No SIM" errors on my US AT&T iPhone 4S by getting a new SIM card from AT&T.
Like many users on the boards, I recently developed an "Invalid SIM" / "No SIM" problem with my new iPhone 4S. I'm in the US, on AT&T, with a black 32GB 4S.
After activation late last week, about every 12 to 24 hours I would lose the AT&T signal bars at the top, the AT&T indicator itself would vanish, and in its place would be "Invalid SIM." (I don't have a SIM passcode.) Other iPhone features through WiFi would still work. For two days, a reset of the phone (holding the sleep and home buttons until the Apple logo appears) would get the SIM and the AT&T back. Today, however, it was stuck on "Invalid SIM" even after several resets. I took the extra step of turning the phone off, ejecting the SIM using a paper clip, reseating the SIM in the tray, and inserting the tray. After powering on, the phone would only say "No SIM"--an even worse problem--and no amount of resetting, powering off, or reseating the SIM would get it back to even "Invalid SIM," let alone working and able to send/receive calls.
I went to the local AT&T store (a real AT&T store, not a partner store or authorized reseller) and they gave me a replacement SIM. I then called AT&T service to have to new SIM activated. I turned off the iPhone, ejected the old SIM, installed the new SIM, and powered on. The new SIM seems to be working as I have the AT&T indicator back and can send/receive phone calls and text messages.
This was a very frustrating problem, tempered somewhat by the helpful messages on this board and a nice AT&T sales rep at the store who was willing to help with no hassle. I have seen other messages where the problem reappears even after a new SIM, and as I have no idea what caused the problem if that happens to me then I will seek a replacement iPhone.
There may be some different or less draconian fixes that work for others, but I wanted to share what worked for me. I also wanted to share that -- at least at the Atlantic Station store in Atlanta, GA -- AT&T was willing and able to solve the problem in under two minutes.