A few days back, iPhone Dev Team’s member announced that a “very interesting” iPhone 4S unlock is in the process of coming out but a recent tweet from MuscleNerd says otherwise:
Baseband Bootrom has now been dumped and therefore the much anticipated wait for this unlock is finally over. What is now being said is that a new version of ultrasn0w might be coming out soon.
It is said that apart from the hardware unlocking solution, there should also be an existing jailbreak in order to unlock the iPhone 4S. So now it’s being assumed that developers have already made a jailbreak solution or we have to wait for the unlocking hack until jailbreak comes out.

I don’t know why it took so long to implement such a basic feature, but it is good to have.