iPhone 3GS is now jailbreakable
You can download purplera1n here.
After you have successfully jailbroken your iPhone 3GS, you can then unlock it using the ultrasn0w tool, provided by the iPhone Dev-Team. However, have in mind that you should stick with iPhone OS 3.0 installed, if you want to do all the aforementioned stuff, since Apple seems to have closed the holes in version 3.1. You can watch videos of jailbreaking and unlocking the iPhone 3GS below, but don’t forget to share your experience in the comments, in case you decide to try it too!
Apple iPhone 3GS Specifications | Review
source: Dev-TeamBlog and CrunchGear via Engadget