Wired has a piece on various potential applications of motion sensors - the latest of which is the Nike shoes/Apple iPod integration.
Why not put a sensor in a hardware token? The something you have would be the token, and the something you know would be some programmed sequence of motions, e.g. left-right-left-up-down.
Might make people look like nervous 3rd base coaches or Catholic priests conducting a Mass. That would be a nice humourous bonus for bystanders.
Or perhaps the sensors will eventually be sufficiently smart to distinguish the owner's gait from that of anybody else? Could it recognize my bad left knee? Recommend appropriate physiotherapy?