And now,bundle for (for both old and new bootrom) has been released. You can use this to jailbreak on the newly released firmware.
And as with all the current4.3 jailbreak releases, this one too is limited to tethered boot only. You will have to use tetheredboot utility to boot your device in a jailbroken state on every reboot/restart.
The instructions for jailbreaking posted here, with appropriate changes for 3GS where necessary, to jailbreak on new and old bootrom iPhone 3GS.3GS are similar to the ones for , and 4G. Simply follow the instructions
Download 4.3.1 for iPhone 3GS
Download iTunes 10.2.1
Download PwnageTool 4.2
Download Universal Ramdisk Fixer
Download iOS 4.3.1 PwnageTool Bundle for iPhone 3GS (Old Bootrom)
Download iOS 4.3.1 PwnageTool Bundle for iPhone 3GS (New Bootrom)