Well now didn’t Apple only just release iOS 4.2 beta to developers? Because it appears that there is available a Pwnage tool bundle for iPhone 3GS on iOS 4.2 beta been released already.
According to an article over on Mobilesdna, iPhone 3GS users can now jailbreak their 3GS with old bootrom using the pwnage Tool for iOS 4.2 for Mac while preserving their out baseband.
Apparently using the old bootrom the user can unlock their iPhone 3GS by using Ultrasn0w 1.1-1 and Pwnage Tool 4.2 makes custom .ipsw firmware files that update the iPhone to iOS 4.2 without updating the baseband.
Mobilesdna has a full blow tutorial on how to get the job done over on their website and is for an iPhone 3GS carrying an older bootrom and has been jailbroken previously using Pwnage Tool, redsn0w, blackra1n or sn0wbreeze and will not work with iPhones that have been jailbroken using Spirit or JailbreaMe.