If you updated to iOS 4 while preserving your baseband, you can now download UltraSn0w from Cydia to unlock your iPhone so it works with any carrier.
In order to have UltraSn0w work for you, you need to:
- have a jailbroken iPhone 3GS
- be on baseband 04.26.08
More info on the iPhwn Wikee. [via French iPhone]