According to sources:
It turns out the new boot ROM doesn’t totally prevent the 24kpwn exploit employed by the Dev Team hackers. [See update below] The result? You can still jailbreak your late-model 3GS, but the device needs to be tethered to your computer in order to boot up. It’s a major annoyance, especially given how crash-happy 3.1 phones—especially jailbroken ones—can be, but not necessarily a dealbreaker.