Not a big surprise really, iPhone Dev Team announced a few weeks ago that they had an unlock ready for iPhone 3GS, but wanted to keep it in their pockets for now, in order to get as many people as board as possible. While this may sound weird, it does make sense after all. iPhone OS 3.1 is about to be released and if iPhone Dev Team announced how they exploited the 3GS and iPhone OS 3.0 software already, there’s a big chance Apple will fix it in time for 3.1 and then we can start over again.
Unfortunately, some other dude didn’t care about that and decided to release his purplera1n software which uses the same exploit as iPhone Dev Team does. So the cat is out of the bag, so to speak.
Using the redsn0w jailbreak will gain access to the file system within iPhone which then allows execution of another program called ultrasn0w which does the actual unlocking of the iPhone.
As always – be sure to read the iPhone Dev Team announcement at least twice, before you start jailbreaking/unlocking your iPhone.