T-Mobile has sent out invitations to a New York press event taking place on July 10. T-Mobile hasn't given any sort of indication of what they will be unveiling, however, the invitation does state that this is "Our boldest moves yet" which seems to indicate they will be doing more than one thing at the press event.
We'll be sure to let you know what T-Mobile unveils, announces, or otherwise makes known at the July 10 event. Doors open at 2pm EST, so you can expect news to start coming shortly after that.
This event could possibly relate to its Uncarrier plans, with rumors of a phase two taking place July 14. It could also be to announce the Xperia Z, currently rumored to be coming on July 17.