The folks over at iPhone Dev Wiki posted a free SIM unlock tool for the iPhone. Similar to the other solutions, this software-only solution should allow you to SIM unlock your iPhone, allowing to use any GSM cell phone carrier. In the U.S. this limits your alternative choice to T-Mobile (over AT&T), but in other countries, GSM carriers are far more common.
More download links and information has been compiled in this forum thread by ascham87.
Apple, iPhone, iPhone Hack, iPhone Unlock, SIM Unlock, Utility, Tool, Software, GSM
Source:→ Mac Rumors