Lumia 950

It looks like the Lumia 950 will be exclusively sold by AT&T in the US. The phone's yet to make its way to the Microsoft Store, where the 950 XL is currently up for pre-order.

A T-Mobile support representative, in response to a customer query, revealed that AT&T has exclusive rights:

The carrier also mentioned that it would not offer Windows Phones directly, although any unlocked devices would work on its LTE network:

We'll have more to share regarding availability over the coming days and weeks, but for now, it looks like AT&T is the carrier to go with if you're interested in picking up the Lumia 950. Given the network compatibility of the phone, it is likely the Lumia 950 will be available unlocked at some point. If you'd like to go with an unlocked option right now, you can always get the Lumia 950 XL directly from the Microsoft Store.

Source: Twitter (T-Mobile)

Thanks for the tip John!