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. [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.
Anyhow, chances are it won't be this way for long—remember the iPod Touch 2G? It was jailbroken fairly quickly after launch, but it had a new, slightly more secure boot ROM, and there needed to be tethered in order to boot. A few weeks later, the hackers finished their thing, and there was moderate rejoicing. This chain of events, which is already under way again, is starting to read like a script.
UPDATE: Evidently, the boot ROM update does disable the 24kpwn exploit, which was the bit of code that allowed for untethered jailbreaks in the first place. For what it's worth, the Dev Team still sounds unfazed. [Gadget Lab]