I've been having the same problem, and it has become a big problem.
Trawling the net, I found a thread on the vodafone site, but relating to the OneX which state's the same symptoms:
When I lose signal due to area, it will stay in that state even when back in a good area. The only thing that gets it connected is switching in and out of Airplane mode. Then all ok, until out of srea again.
Before this, its signal strength has been poor for ages compared to my work Nokia on the same network (Orange). Just put the SIM into my old Desire, and signal is much better.
This is very annoying as when waiting for a call you dont always notice its changed its network state.