UPDATE : The following guide holds good only for iPhone 3GS 3.0 firmware. For 3.0.1 use redsn0w guide instead!
Geohot has done it again! He gave us the first jailbreak for iPhone 3GS and now, we have a much more efficient iPhone 3GS unlock – purplesn0w. Seriously, all the sn0w versions are confusing but well, that is their game!
For now, the iPhone 3GS should be activated using an official carrier before purplesn0w is installed on your iPhone 3GS. (Hacktivation for other carriers to follow soon)
Why is this unlock (purplesn0w) more reliable …