The iPhone 3GS is now jailbreakable and unlockable via redsn0w and ultrasn0w on both Mac and Windows! In the words of the iPhone Dev Team, "Last night we released updated versions of our redsn0w jailbreak and ultrasn0w carrier unlock. These versions are now compatible with the iPhone 3GS running at 3.0. Welcome aboard, 3GS owner!" One can't help but notice a sense of triumph in their words. While the Dev Team was planning to hold out on the release due to fear of Apple releasing an update blocking the hole they used to hack the software, the release of PurpleRa1n
The new version of redsn0w and ultrasn0w are essentially the same but with a few updated tools which allow it to be entirely functional with the new iPhone 3GS. It's all safe, tested, and certainly works if you do it correctly. The recent release of PurpleRa1n, another jailbreak tool, is less stable and buggier than the Dev Team's tools. If you're new to the iPhone world, then let me tell you one thing: always go with the Dev Team.
To download the stuff, simply go to the Dev Team's download page.
Also, do not forget about universal jailbreak BlackRa1n tool for iPhone 3.1.2 from GeoHot (note that this tool will not unlock)
Then simply follow our guides:
30 Second Universal Jailbreak Using BlackRa1n Tutorial: Unlockers Beaware (Tool from GeoHot, we do not recommend)
How To Jailbreak iPhone OS 3.0 On Windows and Mac Using Redsn0w (Tools from DevTeam)
How To Unlock iPhone 3G and 3GS Using Ultrasn0w (Tools from DevTeam)