An eye popping video from YouTube user justkylevids who put an iPhone inside of his guitar and recorded the string vibrations.
I just happened upon this trick when testing what it was like filming from inside my guitar. Note this effect is due to the rolling shutter, which is non-representative of how strings actually vibrate.
The reason we can see the strings oscillate the way they do is because the iPhone 4′s rolling sensor scans items starting from the top left and moving towards the bottom right. The scanning is done very quickly, but when photos or videos are taken of fast moving objects, interesting and distorted photos and videos are the result.
To wit, check out this photo of an airplane propeller also taken with an iPhone.