If you have a formerly software unlocked FW 1.1.1 updated bricked/locked phone, READ!
I want to save you some trouble and time, so please READ this CAREFULLY and don't ask a thousand questions afterwards because I really think it explains everything. If you didn't understand, READ AGAIN until you do!
This information is ONLY valid for those iPhone users who software unlocked their iPhone before they updated to FW 1.1.1 and who succeeded in downgrading to FW 1.0.2, re-enabling activation/jailbreak/SSH install/etc. but are still stuck with the baseband modem firmware from FW 1.1.1!
1. save your time and don't try to downgrade the baseband modem fw from 04.01.13_G using the currently available instructions using bbupdater and the .fls and .eep files. It won't work, you can try as much as you want, you still end up with a .fls download verification error. Yes, I tried DOZENDS of variants of this method, involving lockdownd, with and without SIM, etc. ect. ect. Don't waste your time, it doesn't work this way.
2. if you previously had a TurboSIM configured (before you applied the software unlock), it won't work anymore. Don't waste your time trying it, your TurboSIM is very likely OK, not broken, it just doesn't work since the software unlock and updated FW 1.1.1 messed up things.
3. if you have an official AT&T contract with official (former) activation of your iPhone, it still won't work with the AT&T SIM and you can't officially activate your phone again. Don't waste your time going to the nearest AT&T shop (like the iTunes message suggests), even a new SIM won't work as long as the baseband modem fw is still the one from FW 1.1.1.
Now the good news: since, thanks to Kmac, we are able to activate/jailbreak software unlock bricked phones (updated to FW 1.1.1) again, it is only a matter of time until a solution for the baseband fw downgrade will be available. So PLEASE be patient and don't ask the same stuff over and over again. Follow the discussion very closely and remember that every couple of hours, there is some new information available and you need to READ this new stuff FIRST before starting to ask questions again. Thanks.