Dhavalis build this Netra project, an ultrasonic eye for the visually impaired:
Netra is an eye for visually impaired, a device based on Echolocation principle. It uses Arduino Micro, Ultrasonic sensor and vibrating motor. Vibration of the motor increases with decrease in distance and vibration decreases with increase in distance between device and object in front. Build this device for people suffering from Retinitis Pigmentosa Disorder.
Via the Arduino blog.
nice, i had a shot at making one of these a few years ago.
http://blog.samcuttriss.com/?p=359
interestingly after doing it i was looking for people doing the same, turns out there is a guy from new zealand doing a much more resolved one: http://www.engadget.com/2005/04/07/bat-k-sonar-cane/
looks like he must have closed shop though.