If you've been following the Galaxy S III news today, you know it has a banging new Exynos 4 quad-core processor that absolutely obliterates benchmarks. The problem is that the Exynos 4 platform is quite old at this point (for a mobile chipset), and was never designed to support LTE. That's why devices like the Galaxy S II Skyrocket don't use an Exynos chip. Devices with Exynos 4 chips that do, like the Galaxy Tab 7.7 LTE, use an external one - adding thickness and increasing power consumption.
While Samsung has hinted at times that the Exynos 4412 chip could, in theory, support LTE with an external (read: adding thickness, weight, decreasing battery life) modem, nothing concrete has come of those rumblings. It also seems fairly unlikely that Samsung would design an entirely different version of the 4412 chip just for US phones.
Given that the global versions of the Galaxy S III will not support LTE, and that the global version will have Exynos 4 Quad, it's pretty easy to pick apart why Samsung is trying so hard to obfuscate exactly what chip will be powering handsets in individual regions. In fact, Samsung says in their press release that regional versions of the Galaxy S III "could" be using alternatives to the quad-core chipset, and when a company admits something like that "could happen," we all know it really means "will happen."
This has one of two outcomes. Either Samsung is planning on introducing its Exynos 5 dual-core platform in the US, which is based on ARM15 architecture - this is the more unlikely option, since it's not scheduled for production until Q2 2012, aka right now (usually there's a delay from production to introduction in hardware) - or we'll see Snapdragon S4-powered Galaxy S IIIs here in the states. The Snapdragon S4 is more likely for two reasons: first, it's already in production and been proven in practice to work with LTE in HTC's One XL (aka the AT&T One X), and second, the LTE chip is integrated on the main board, meaning no compromises on thickness or weight, and much less so on battery life.
One exclusion to this "rule" may be T-Mobile, who have yet to set out a timeline for their LTE network rollout. It seems exceptionally unlikely they'll have one rolled out in time to make launching the Galaxy S III with LTE a remotely feasible (or intelligent) option, and so they may instead opt for the 21mbps HSPA+ Exynos Quad version of the device, making them the sole carrier of the "true" Galaxy S III in the US.
It's all speculation at this point, but it's grounded in pretty solid evidence, and for the aforementioned reasons, we're pretty confident Exynos Quad won't be showing up on at least three of the "Big Four."