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. Previous phones used the much harder to modify zImage kernel. There is a recovery partition available in this firmware that should allow recoveries to be flashed separately from the kernel. There is, however, a counter in the bootloader that detects the modified kernel, but that is par for Samsung devices and doesn't negatively affect things.
Chainfire has decided against releasing the insecure kernel for the time being. He worries that the build he is working with could be traced back to the individual that leaked it. Additionally, this was all done with release candidate software, and things could technically change in the final firmware for devices. Carrier-specific versions of the Galaxy S III might also be more locked down.