No support for iPhone 3GS and 2nd/3rd gen iPod Touch devices
The iPhone-Dev team has released Pwnage Tool 3.1 for Mac OS X to enable iPhone OS 3.1 jailbreak as well as unlock. The Pwnage Tool will enable jailbreak for first generation iPhone, iPhone 3G and first generation iPod Touch. However, the iPhone-Dev Team has said that currently the Pwnage Tool 3.1 won't support iPhone 3GS, second gen iPod Touch and third gen iPod Touch.
The iPhone-Dev team cites that Apple's update technique has led them to push out multi-part release of iPhone OS 3.1 jailbreak. In case you don't know, to use certain features of iTunes 9 like Genius Mixes, App Management, et al, the iPhone OS 3.1 update is required on the connected iPhone OS-based device.
Users (iPhone and iPhone 3G) who have ultrasn0w on their phones should first create a custom .ipsw file using Pwnage Tool and then upgrade their devices with the new custom .ipsw file. The iPhone 3G users will need to continue using ultrasn0w with Pwnage Tool created .ipsw in order to apply their phone with iPhone OS 3.1 update. First generation iPod Touch users will get to use firmware 3.1.1 update for creating a custom .ipsw with Pwnage Tool.
The redsn0w tool for Windows and Mac OS X will be released some time in the near future. Hence, the cat and mouse game between Apple and iPhone Dev team continues.
Note: Using Jailbreak, unlock tools will void Apple's warranty for the respective device and may render the device unusable.
I would imagine they have the 3gs JB ready but are waiting for the 3.1.1 MMS update to be released on the 25th first so they do not expose the hole to apple. I have no inside knowledge, but that makes sense to me.