Even though the device hasn't even hit the street, noted Android developer Chainfire has obtained root on the Samsung Galaxy S III. Chainfire doesn't actually have the device in hand, so don't start berating him with questions on that matter. Rather, he got root on a firmware build that was leaked to him, and has a few juicy tidbits to share with everyone.
It appears that the Galaxy S III isn't going to be locked down in any significant way. All Chainfire had to do was repackage the kernel with a modified adb binary and install SuperUser manually. Samsung chose to use the standard boot.img kernel format as well.