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.
The software is being mirrored (Gizmodo mirror) by a number of locations along with downloadable source code. Tutorials are still being developed but some early instructions have become available.
More download links and information has been compiled in this forum thread by ascham87.
Source:→ Mac Rumors