Available for download now. If you want to jailbreak the iPhone 3GS and have a Mac, you can now do so. Interestingly, we are hearing that Ultrasn0w, the SIM unlocker for iPhone 3G from the iPhone dev team also works for iPhone 3GS (in many cases).
Once you have Cydia from PurpleRa1n, you can install UltraSn0w from there. The whole process takes about 15 minutes.