It's probably not very practical for higher speeds. In order for something like that to work, you'd need to be using the CMOS sensor in the iPhone in "video" mode, where it's constanly exposing and "waiting" for the light to hit the sensor. And since the frame rate for iPhone cam is 30fps, reallly the fastest shutter speed you can measure using that is 1/30s. I'm guessing that the Shutter Speed app you have can go faster than that?