Update: This guide also applies to iOS 4.3 final version.
After releasing PwnageTool bundles for iPhone 4, iPad and iPod touch 4G / 3G, iPhone developer and hacker iH8sn0w has now released bundle for iPhone 3GS (new and old bootrom) which can jailbreak this device on the newly released iOS 4.3 firmware.
And as with all the current iOS 4.3 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 iPhone 3GS are similar to the ones for iPad, iPhone 4 and iPod touch 4G. Simply follow the instructions posted here, with appropriate changes for iPhone 3GS where necessary, to jailbreak iOS 4.3 on new and old bootrom iPhone 3GS.
Download iOS 4.3 GM for iPod touch 4G / 3G
Download iTunes 10.2
Download PwnageTool 4.2
Download iOS 4.3 PwnageTool Bundle for iPhone 3GS New Bootrom
Download iOS 4.3 PwnageTool Bundle for iPhone 3GS Old Bootrom
Download iOS 4.3 PwnageTool Bundle for iPhone 4
Looking to jailbreak iPhone 4 on iOS 4.3? Follow the complete step by step instructions posted here to jailbreak iPhone 4 on iOS 4.3 using PwnageTool.