Even though the Samsung Galaxy SIII is not yet available, that did not stop a developer . This was done using a leaked software build and did the rooting virtually. Samsung does not seem to lock down the Galaxy S III in any significant way because Chainfire had to do was repackage the kernel with a modified adb binary and install SuperUser manually.
Even though Chainfire has achieved root he has decided against releasing the insecure kernel for the time being. Reason being it could implicate the individual who leaked it. Further more this was done with with an older release candidate, therefore the version you will receive from your carrier may differ.