Hi Folks
Here is a link of some students that used it for some or other gravity experiment in the Vomit-Comet. I am not actually sure how useful it was, because there was no mention of the iPhone results in their final report.
Accelerometer Data
And if you go to the forums for manufacturer of the following app, they do discuss using an iPhone for propeller balancing.
Showcase: Vibration Turns Your iPhone into a Spectrum Analyzer | MacResearch
Software
Cheers
Ed