iPhone 3GS Unlocked!
This is kind of old news - but I realized a lot of folks may not have put two and two together.
ultrasn0w has been out for a while, and has always been known to work on the 3GS "as soon as it was jailbroken" so it could be installed - how could you install it without a jailbreak.
And now, purplera1n has jailbroken the 3GS for both Windows and OS X users.
So how do you unlock?
Easy - jailbreak from either Windows
or OS X
, then install ultrasn0w