There is now a simple way to enable tethering on the iPhone 3G/3Gs using Optus without paying extra. Now if you do happen to get charged I take no responsibility.
Your iPhone must be JailBroken. Add http://apt.iphone-storage.de as a source in Cydia, then search for Tethering and install the one corresponding to your iPhone (Tethering 3G – 3.1.2 or Tethering 3GS – 3.1.2).
Once installed, reboot then open Safari and navigate to http://justynsmith.info/mobileconfig/au_optus.mobileconfig and install the mobileconfig file. Once installed you should be able to go to Settings > General > Networking > Tethering (on/off).