EPIRBs

Very briefly, an EPIRB (Emergency Position Indicating Radio Beacon) is a radio transmitter which transmits a signal that is monitored by satellite and civilian and military aircraft.

The type of EPIRB 4X4 drivers and bushwalkers are most likely to purchase operate at 121.5/243 MHz.

Unfortunately it seem that satellite monitoring will cease after 2009 although aircraft monitoring will remain making them still a worthwhile device.
They are still a better bet than mobile phones in most remote areas.