Thanks to a new app that uses the same approach as submarines for measuring distance, you can now add 'semi-reliable tape measure' to the list of things your iOS devices can do.
Acoustical Ruler is available from the iTunes App Store for just a buck, and uses simple sonar techniques to measure distances with a couple of different approaches. In Dual Device Mode it's able to measure as far as 25m by "clocking the time delay" of an emitted chirping sound between a couple of microphone-equipped iOS devices, as is demonstrated in the video. If you don't have a couple of iOS devices, the app also has a Single Device (Headphone) Mode which works with a single iPhone, iPod Touch or iPad using a pair of earbuds. The only drawback being that you're then limited to measuring distances as long as the length of your earbuds.
The app requires a bit of calibration each time, including specifying the air temperature in the room since that will affect the speed of the sound waves. And while it's claimed to offer a resolution of just one-tenth of an inch, the developer makes no promises of how accurate it really is:
Please note that while the resolution of the measurements may be as low as 1mm, the precision usually is not. While I have taken great care to make the app as reliable as possible, there are simply too many factors affecting the measurement process and the precision. That is why I want to be clear about one thing: there is absolutely no warranty that the measurements taken with Acoustic Ruler Pro are correct. Since the app works by sending and receiving sound, you'll obviously get the best results when using it in an environment that is not too loud.