I found this ......seems to imply the Trusted USB Hub is only for the keyboard & mouse which may be a SSC chip that either lies in the keyboard or the MB or somewhere in between.If it was in the keyboard you would presumably have to plug the mouse into the keyboard.....just a few thoughts:)
"Input is secured by a trusted USB hub, for the keyboard and mouse, that carries on a protectedconversation with the nexus. Output is secured by a trusted GPU that carries on a crypto-protectedconversation with the nexus. This gives us "fingertip to eyeball security"