Your mobile devices have secrets, which is why you should require an unlock code before each use. But if you're especially nervous about onlookers seeing your screen over your shoulder, the same kinds of laptop privacy screens are also made in mobile phone sizes.
While I normally want LCDs to be readable from wide angles, these overlays restrict the perspective on the periphery, leaving things mostly the same in the center. I tried one from 3M on my iPhone, and it works as designed, although it adds a little bit of texture that I didn't mind. People on the sides of the device can't see my passwords or read my email, just as designed. The main drawback is that it slightly darkens the screen overall.
Just be sure to get one with the proper orientation. The screen fades the side view to black, but it doesn't alter the vertical perspective. This isn't a problem on a device like a Blackberry, but the normal privacy screen has little affect on an iPhone in wide mode. So airplane gawkers might still see which reality TV shows you secretly watch.