T-Mobile is the smallest US carrier, and it was also the last to announce a cogent strategy for the deployment of 4G LTE. Yes, after years of insisting to no end that HSPA+ is 4G, the magenta carrier is rolling out LTE. As part of that move, new phones are going to be needed. The first device designed for T-Mobile's LTE is a revamped version of the popular Samsung Galaxy S III.
T-Mobile has not yet said when the updated S III will arrive, but it will likely replace the HSPA+ GS3 in the carrier's lineup. Most of T-Mobile's phones have no support for LTE, thus the new version of Samsung's flagship.