STM is their new type of motor, being launched tomorrow with the 40mm pancake and the new 18-135.
S is Silent, M is Motor, the T i'm not so sure, probably Transmission?
Either way, it's not a USM because that requires all the Ultrasonic bits. From what I gather, it's just a normal micromotor, just Silent (for whatever "silent" means, I can hear a USM if my ear is close enough...)