You would think a necessary component for rooting the upcoming Samsung Galaxy S III would be the actual handset. Unless you're exceptionally pro, in which case you can just do it remotely using a leaked kernel. Before you ask, yes, this exact feat has been accomplished already, so the day you do purchase your shiny new S III, it's almost guaranteed you'll be able to root it immediately.
Ausdroid picked up the news yesterday, though at that stage the insecure kernel was not provided in the original xda-developers post, as "Chainfire" (the author) was concerned it could be used to track down the person who leaked it. Now, it appears that's no longer an issue, as the post has been updated with a link to the kernel.
Not that it's a big deal anyway as the instructions on rooting the kernel, while not "noob-proof", sound fairly straightforward for someone with a bit of know-how.
Chainfire mentions the possibility of an updated firmware being present on the retail Galaxy S III, but believes whatever changes are made will have little effect on the ability to root the device. In his or her words, Samsung is the only "high-end Android OEM who aren't [sic] complete douchebags in the unlock department!"