Might be worth a try, i've played around with a Microsoft mobile version on a PDA before in an aircraft, but not on a Flight Trial. How do you know/ can quantify what the iphone is actually telling you, how accurate is it recoding vibrations, probably as good as the App; and was that made for just a bit of fun or did the developer consider a more serious type of application?
A quick ball park check maybe but nothing more accurate unless you know exactly what the iphone is telling you before you start.
I might think about (sorry if you have already) for the iphone approach
What’s the vibration range (in Acceleration, Displacement, Velocity)
What’s the accuracy ( full scale %)
I don’t have an iphone but does the screen lock (stopping the program) battery life, mounting etc etc.
Hope that helps