First of all, I never had any problems with the iPhone 4 before the upgrade of the OS to OS5
Secondly is Apple aware of the problem? because this thing is driving me crazy.
I have read many things online and the ones that work for me are the followings.
Settings -> General -> Cellur Data (Turn it On and Off)
Settings -> Airplane Mode (Turn it On and Off)
This is a rather odd fix so Apple please fix this crap.