I would not expect them to update the 15-85mm any time soon.
The 15-85mm lens is just about the same price as the T4i itself, and lenses at that price point are not appealing to most Rebel/xxxD shooters. So, likely no need for STM in that lens. Maybe if Canon adds video AF to a higher end body, we'll see more STM lenses down the line...maybe. But there's a trade-off - STM lenses focus more slowly than their USM counterparts, and that's going to be a turnoff for many users of high end lenses. The 18-135mm was not USM anyway, but micromotor - STM made is smoother without a focus speed penalty.
Optically, the 15-85mm is already delivering L-series performance (though without the L-series build). Given that, I'm not sure there's much of a need for an update.
Thank you thank you, that was extremely helpful.