IQ-Tainment has a really cool iPhone hack. They have developed and app that emits acoustic waves that measures the distance between two phones or between the phone and a microphone. The measurement may not be accurate, but it is a fun app nevertheless. To make it work, place the iPhone or iPad on one end and use the app to emit a sound.
Gadget.com
See also:
techcrunch, Use iPhones as Acoustic Rulers
