Thanks for the positive feedback, I'm looking forward to your test results
@rich815: well, what exactly is your problem ? you might have a look on the app-website I linked in my post: http://www.echolot-online.de/Shutter-Speed.html
Of course it is only an approximation. But in most cases, it's a quite good one. I compared the values from this app with an optical measurement (photoresistor + audacity) and till the 1/250 the results are quite accurate, the deviation is not more than 1/3 f-stops. Of course this also depends on the camera you are using.
This app isn't designed to measure the shutter speeds at microsecond-level. You won't be able to tell if the 1/250 is more likely a 1/220, but this isn't really necessary as you don't see the difference in your final image anyway. What you can detect, are larger deviations, wich really affect your image. For example if your 1/125 is rather a 1/80.
I had the same idea with the light-sensor, but unfortunately you can't read this sensor without the use of private APIs (forbidden commands). What you could do, is attaching an phototransistor to the mic-input of the iPhone, but I haven't tested this yet.