Mobile Phones
Most riders have them, and most of us carry them. Coverage is reasonable along much of the Great Divide, with reception available from some surprisingly remote hill tops. Choose your carrier though. Telstra still has the best coverage, while some of the others are almost useless outside of the major centres. The choice of phone also makes a difference. Some units are built for city use and have low power transmitters, while others were made specifically for the Australian Next-G roll-out with considerably higher signal capabilities. Check it out before you sign up.
Mobiles are mostly useless in the desert unless you are very close to a town. Again, Telstra will give the best chance of getting a signal in the spinifex.