HTC is keeping their promise of bringing KitKat to carrier versions of the HTC One in the shortest time possible. Both AT&T and T-Mobile has given the respective firmwares technical approval, so it is only a matter of time before an OTA notification light up your One.
The update will bring Android 4.4.2 KitKat with Sense 5.5 UI. Sense 6.0 UI will arrive at a later date, in a separate update. Meanwhile, Sprint is nearly a week into rolling out KitKat for its own HTC One variant.
T-Mobile HTC One Owners! We have received technical approval for 4.4 OS + Sense 5.5! Expect the OTA out shortly! Thank you for your patience— Mo Versi (@moversi) February 21, 2014