iPhone 3G only: The iPhone Dev Team responsible for jailbreaking tools like QuickPwn and PwnageTool release the unappetizingly-titled Yellowsn0w, unlocking software that makes using a different carrier on your iPhone possible. In theory. Gizmodo reports that the unlock didn't work on their handset, as well as a significant percentage of other users (though it did for many, too). To try it out, take a deep breath, jailbreak the phone, and install yellowsn0w via the Cydia or Installer app.
Did you give the unlocker a try? How did it go for you? Post up your experience in the comments.