Something I have always been wanting to do, more likely a friend since I do not program, is to bring a GPS unit and a valentine one together. The V1 already determines direction, by writing something that can estimate time between the read and the return signal you can estimate distance. Then calculate your position, GPS already does this, then apply that distance and put a ping on the screen of where it mostly likely came from.
You could use the idea of the EDGE iPhone (some what 3G iPhone), where you have the pin and then a light blue circle flashing of the possible location around the pin since it is not 100% accurate.
Note sure if this is possible, but working in under water survey and with sonar has brought that idea to life, just a different use for it.
