The hack was accomplished by modifying the iPhone's default SIM card to be recognized as an AT&T SIM, while actually containing data that allows it to be accepted by Telstra.
The hack is made possible through the use of iActivator, a Mac OS X program that provides an interface for activating/deactivating your iPhone without invoking an AT&T contract or prepaid plan. The program doesn't unlock the iPhone's SIM, but simply unlocks the functionality of the iPhone so that an AT&T-based activation is not necessary. The tool also has the option to perform a "Jailbreak" on the iPhone, which allows files to be written to the device's filesystem.
Here you have the details:
1. Get the required hardware and softwares: (these are the ones I have used): An Infinity USB unlimited SIM reader/writer, a silvercard, SIM-EMU 6.01, and WoronScan 1.09
2. Get the IMSI, Ki of your carrier using WoronScan (I will call them IMSI-b, Ki-b)
3. Use SIM-EMU and create 2 files (1 Flash and 1 EEPROM) using the ICCID of the AT&T sim (ICCID-a), IMSI-b and Ki-b
4. Then use these 2 files to create a sim using the infinity usb unlimited reader/writer
5. Put this sim into a normal unlocked phone and make some calls/receive calls/data services
6. Then use SIM-EMU to change the IMSI of the original Flash file to IMSI of AT&T sim (IMSI-a)
7. Again write the silvercard with the new flash and eeprom files
8. Put this sim into the iphone
9. Activate using the Cingular method as descirbe in www.Hacktheiphone.com...