Following up on the 3.1.2 jailbreak news yesterday, members of the Dev-Team have confirmed that Apple has begun shipping iPhone 3GS units with a new bootrom that fixes the 24kpwn vulnerability that allows for jailbreaking as of last week.
Because of the new bootrom, the team has confirmed that a standard jailbreak would be impossible until a new vulnerability was found, as all current jailbreak tools are based off of the 24kpwn exploit being present on the device.
The team goes as far to suggest to locate older models and/or refurbs of the 3GS in the meantime if one wants to jailbreak the iPhone until a new exploit is found.