Hello, I'v got same problems with the 3GS in combination with T-Mobile. My wife also with her brand new iPhone5!! Sold today from Vodafone.
Since iOS6 signal strength is hopeless. It bounces all the time, being far from stable.
Apple please quickly release a path and return service to normal stability.
I am having something similar, but for me is not one bar. that is 5 bar full (which seems not really make my phone reception weak, rather it go random).
I am now in a situation that make me frustrated, it can be having prefect signal, it go weak, No Service and come back. After a while it repeat. This is in good situation after I reset all setting. hopefully it will not go even worse.
before I reset, after it go No Service, the phone seldom come back, it stay for no service for quite a long time, can be for whole night, until I restart the phone. Because of this problem, I also tried restore my phone, which dosen't change anything.
Now I would love to know some fix that can get this problem away, or at least we know in what situation that happened, then we can try to avoid.