The only time my upper-body has received a workout from my bocks was when I hurt my ankle early in the day, then spent almost an entire day's jam carrying them around. But I guess it depends on how built you are to begin with. If you're not used to any kind of upper body work out to begin with... I suppose using your arms to jump would make them ache somewhat... but not if you're used to jumping around. Otherwise I'd say it's just from the carrying.
But there's no arguing about the legs and core. Even experienced mountain bikers will feel the burn after a long day on bocks! ^_^