iPhone 4 doesn't regain service after going into subway

About two weeks after updating my iPhone 4 to iOS 5, I started getting an issue where I never regain reception after coming out of a subway tunnel or an area that loses reception.

I use the subway to commute to work and every time I enter the tunnel I, obviously, lose reception, but when I come out, my phone just says it's searching constantly but it never finds a signal. The only way for it to find a signal is to reboot it. I never had this issue until a few weeks after iOS 5 was released.

Does anyone else have a problem such as this? Is there some kind of solution to this issue?