Mac OS X only: If you're running a 2G or 3G iPhone with the 3.0 firmware, your (unofficial) carrier options just opened up a bit, as the Dev Team has delivered Ultrasn0w 3.0 for full unlocking.
Downloading and installing the unlocking tool requires that you have jailbroken your phone with PwnageTool, which means that, at the moment, it's only available for Mac OS X users with 2G or 3G iPhones, and not the new 3GS—though the Team is working on both Windows tools and 3GS access. Still, if you've been intrigued at the idea of switching, now you can do so with a 3.0 phone.
Found success with unlocking your iPhone in the past? Wondering who in their right mind would go through this just to pay one exorbitant data plan over another? Let's hear your thoughts in the comments.