Thanks to an unaddressed security flaw, the iPhone Dev Team have announced that they will soon update their ultrasn0w unlocking tool to work on the iPhone 3GS.  The exploit, known as 24Kpwn, was first discovered in the iPod touch 2G, and used in the redsn0w tool to jailbreak that PMP.

Now, the Dev Team can use the same sort of technique as used in redsn0w to update ultrasn0w for Apple's latest smartphone.  The team believe that the iPhone 3GS' bootrom was finalized in roughly August 2008, so the emergence of 24Kpwn earlier in 2009 did not affect it.

The Team are now working on integrating the new technique into ultrasn0w, which they've warned will "take some time".  Once that's done, however, it will be possible to both jailbreak and unlock the iPhone 3GS.