iPhone4 has amazing camera and you can shoot 720p video @30fps with it. A Youtube user captured the vibrations of his guitar strings using his iPhone4’s video camera. He placed his iPhone4 in the camera mode inside his guitar and played a tune. He plays some tunes by Eric Clapton: Tears in Heaven and Signe. Amazingly enough, he could capture the oscillations of the strings very impressively.
The uploader “justkylevids” has given some tips for capturing videos in similar way. He has suggested to get the strings brightly backlit so that the camera captures at a high frame rate; for that he has suggested to use a pencil. This video was captured with iPhone’s front facing camera and the uploader has suggested that the back camera may capture better. This is really a neat idea and the uploader shows how the vibrations produced by guitar strings can be easily captured using a camera phone.