Fear not, people who jailbreak your 3GSes! PwnageTool for the latest firmware, 3.1.3, is available and ready to crack, hack, and otherwise damage your iPhone.
I’ve stopped hacking my 3GS simply because too many things break after unlock. You should also keep your “SHSH hashes” backed up (basically keep the keys to previous versions lying around so you can revert. If you don’t know what it means, don’t upgrade.)
Otherwise, rock out, friends. 3.1.3 awaits.