Let me share my thoughts with your regarding the iOS 5.1 "fix".
I sincerely believe Apple didn't really fix the issue, but just found a away to work around it... and that isn't really as elegant as one would hope.
In the places where I'd previously be more prone to have the Invalid SIM/network problems, I have now found that I sometimes get recurrent "network reboots" - as seen by the signal strength bars going from full to zero, and back to full, in a continuous loop.
Meaning, I think the "fix" Apple implemented is just: when they detect the network drop problem, they "reboot" it, just like we previously did manually, by either turning Airplane on/off, or rebooting the iPhone.
For the most part... it should have the intended effect - although it doesn't really address the *real issue* of what might be causing this behavior in the first place. But, just like before, I suspect this is an issue that keeps becoming more frequent over time. Just yesterday, I once again received a "tried to call you" SMS, and when triying to call back - even though it showed full signal strength (and not being in a problematic area) - I couldn't even get a call signal, and had the 3G data indicator jumping back and forth to "none", "circle", "none", "circle".
Just like before, my iPhone was showing full bars, but was effectively unable to do voice calls (either incoming or outgoing). I had to manually set it to airplane mode and back again, and then it finally got "online".
I fear this will become increasingly more frequent, just like the issues we had before the 5.1 update... and that the problem with the 4S isn't really ever going away. Which is a real shame, considering all the previous iPhone generations never had this sort of problem and could handle every kind of SIM card without any issues, as well has being "bullet proof" to all kinds of network conditions. Looks like they've gone a step backwards this time.
I have had a black iPhone 4s 16 GB for almost 6 months now (I got it near the end of January 2012) and I've never run into any SIM card issues. Even on iOS 5.0.1, I've never had issues with my SIM card. I'm in Canada on Telus Mobility for all those wondering.
My friend just got a black iPhone 4 16GB, and as soon as he puts his Telus SIM into the iPhone 4, it says invalid SIM. His iPhone 4 is running iOS 5.0.1, and reading up on these posts makes me wonder if I should try to update his phone's firmware to see if that fixes his issues.
Iphone 4s Issue with IOS 5.01 doesnt happen straight away so might happen later on for you or could be that your one of the lucky one and got it all ok for you
Iphone 4 - Invalid sim - thats a different issue, I have a feeling that his phone is either locked, blocked or sim card is not 3g or something.
Check another sim card in your friends iphone4 and see if there are any other ones that work. And see how many other networks work because it could be that it wants unlocking.