We know the Galaxy S III is coming to the US later this summer, and on more than one carrier at that. Given Samsung's generous nature with the last two Galaxy S models, a release on all four major carriers (and regional carriers to boot) is not out of the equation. TmoNews thinks they've spotted the Galaxy S III's magenta model number in an online video: SGH-T999.
For reference, the original T-Mobile Galaxy S Vibrant was the SGH-T959, and the Galaxy S II was the SGH-T989. (If T-Mobile ever releases a T1000, we're all in serious trouble.) Given that T-Mobile and Samsung are at least as close as any other US carrier, we'd be downright surprised if they didn't get a version of the phone eventually. And if anyone in the US is going to get the quad-core Exynos version of the phone, it 'll be T-Mobile, since their LTE network won't begin rolling out until next year at the earliest.
The only carrier that we can say with any certainty will get the Galaxy S III is Verizon, thanks to a similar leak in a user agent profile. And AT&T's cancellation of the Galaxy S II Skyrocket HD certainly implies that they're looking forward to something bigger and better. In any case, it'll be at least a few weeks before we hear anything - the Galaxy S III won't land overseas until May 29th. If you can't wait any longer than that, you could always give an absurd amount of money to Expansys to import one.