Having the same issue! I upgraded 2 seperate iphone4 last night. One is fine, the other is not. Of course the one that isn't working is my wife's!
Her phone was on 5.0.1 Semi-tethered jailbreak. I installed Corona over that and then found no carrier name listed on the top status bar. The phone signal would appear for a minute, and then disappear. While it is showing you can make a call or receive a call. When it isn't showing, the phone service is dead.
Knowing that I shouldn't have put Corona on top of the semi-tethered jailbreak, I did a restore to 5.0.1. Then I did a restore from iCloud. Everything is fine at this point. AT&T is showing up and phone works (this is not an unlocked phone).
Ran Redsn0w 0.9.10b3
installed appsync 5.0+ , MultiCleaner, Five Icon Dock, and I think that is it. Thought all is well until I handed the phone to my wife and we were out the door. No AT&T logo appears at the top. The phone signal bars are still there and we can make/receive calls/texts. Not sure what is going on.
Will try carrier setting reset when she gets home.
Any ideas what is going on?