For the last several months, anyone who had updated to iPhone OS 3.1 or greater without following a pre-update procedure lost the unlock on their iPhones due to new software in the modem firmware or the baseband. Using the at+xemn crash as an injection point, iPhone hacker Geohot (of the original hardware unlock) was able to unlock the latest firmware on any iPhone.
What does this mean? The iPhone can be unlocked for any GSM carrier, allowing you to use it on more than your country's authorized carrier or other carriers as you're traveling abroad.
Geohot suggests that you update to a fresh copy of iPhone OS 3.1.2 directly from Apple before performing his unlock. He has all the instructions available on his site.