While T-Mobile announced that they would support the Moto X, they won't be giving it the attention that other big carriers are providing. Namely, T-Mobile won't be selling the new device in their stores or on their website. Instead, buyers will have to go to Motorola's website to buy it if they want it on T-Mobile's network.
T-Mobile's chief marketing officer, Mike Sievert, said to AllThingsD that "the Moto X is optimized for T-Mobile’s 4G LTE network," but the carrier does not have plans "to stock Moto X devices immediately." However, T-Mobile is "working closely with Motorola to make the Moto X a great experience for T-Mobile customers."
This also poses another question about the Moto X: off-contract price. Since T-Mobile got rid of contracts, customers are now needing to buy their phones outright at full retail price, paying either all at once, or spread out over 24 monthly payments. Motorola hasn't announced the off-contract price yet, but we should be hearing about it in the near future.
We're not sure why T-Mobile isn't selling the Moto X in their stores or online, but it could be because of the customization options of the phone, and just going to Motorola's website may make that easier on T-Mobile.
The Moto X will sell for $199 after signing a new two-year contract, and it'll available on the five major US carriers, including US Cellular. At this point, it's up in the air how T-Mobile will sell the Moto X, but the off-contract price should surface before the phone's official launch.