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.