Since iPhone got jailbroken late last week, a few sites and users can confirm that ultrasn0w unlocking tool also works on the new unlocked iPhone 3G S.
You just need to jailbreak the iPhone 3G S and via installer you can install ultrasn0w and you are good to go. Now all USA AT&T users will be able to enjoy the benefits of having a compass. [Something we all need on a daily basis.sub.ed.]
iPhone 3G S is definitely much more responsive than the iPhone 3G and it does work faster, but some user do complain about inferior battery life, and the fact that it cannot work long between charging.
Here is the guide how to unlock the iPhone if you would like to.