iPhone 3GSes now ship with a chained-down boot ROM, intended to thwart jailbreakers at a fundamental level. It's already been cracked. But for what it's worth (seriously, what?), it has made life outside the App Store a little less convenient.
It turns out the new boot ROM doesn't totally prevent the 24kpwn exploit employed by the Dev Team hackers, but instead just kind of interferes with it. The result? You can still jailbreak your late-model 3GS, but the device will need to be tethered to your computer to boot up. It's a major annoyance, but not necessarily a dealbreaker.
Chances are it won't be this way for long anyway — remember the iPod Touch 2G? It was jailbroken fairly quickly after launch, but it had a new, slightly more secure boot ROM, and needed to be tethered in order to boot. A few weeks later, the hackers did their thing, and there was much rejoicing. Cue the remainder of this exact chain of events..now. [Gadget Lab]