A leaked pic of what is supposedly an internal website for T-Mobile indicates that when T-Mo gets the Moto X, it will only be sold through the Google Play Store. The phone has already launched officially for both AT&T and Verizon, which leaves the other two major US players in limbo at the moment. Even this new leaked info doesn't reveal any details about when the Moto X will come to T-Mobile.
This intel obviously infers that the only way you will be able to get the Moto X for T-Mobile off-contract. Of course, T-Mobile has their Uncarrier, non-contract plans now anyway, so this point is moot. It leaves it up in the air whether T-Mobile will allow you to pay out a Moto X over time as they do with other phones. Perhaps if you do sign a payment plan agreement with T-Mobile they will give you some sort of digital voucher to get the Moto X from the Google Play Store. That is just speculation at this point though. We will bring you guys any further details when we find them.
How many of our members want to get their hands on the Moto X with T-Mobile?
Source: Phandroid via MotoXForums