Not that anyone ever had any doubts that the Dev Team would be able to work their magic on the 3GS, but this is quick: within a week of the phone's launch, the hackers say they've figured it out.
The Dev Team says that the 3GS is vulnerable to the same "24Kpwn" bootrom exploit that plagues the iPod Touch 2G—an patchable, un-updatable route of access for jailbreakers. Additionally, they expect to be able to unlock the phone with the ultrasn0w tool, using familiar techniques.
What seems a little odd about this is that the 3GS would so susceptible to hacking, and in such way that had been identified months ago. The Touch 2G had additional protections that the iPhone 3G didn't, and it was release just a few months after. The 3GS, however, was release well over six months after the Touch 2G and quite some time after its predecessors exploits had been publicly identified, but doesn't appear to have substantially different protections.
There are two possibilities here: either the 3GS development and manufacturing schedule is longer and more rigid than the iPod Touch 2G's (the Dev Team says its bootrom was "cut" in August of 2008, before the 24kpwn exploit was identified) or, more compellingly, Apple just doesn't really care that much about jailbreaking anymore. I'll assume the former, and quietly hope for the latter. [Dev Team—Thanks, Bhoumik and Eric!]