It's the fourth day of a scientific conference in Denverfour busy February days in a huge rabbit-warren convention center with long hallways and fluorescent lighting and serious scientists giving serious PowerPoint presentations in darkened auditoriums; four days of breakthroughs and advancesnanotech to biotech, anthropology to zoology, the whole mind-spinning stew. Four days, for the assembled journalists, of making sense of it all and banging out stories on the fly-and now comes word of what could be a light interlude: Keep an eye out for the guy carrying the head. Say what? The robotic human head.