Around the Viacom lunchroom, candy machines, and magazine stand, David Greg Harth is a pretty famous guy. The preppy 23-year-old graphic designer-cum-conceptual artist has singlehandedly managed to circulate 10,000 dollar bills stamped with the phrase I AM AMERICA in the past six months. Getting the bills out there was a painstaking process until last week, when Harth hit pay dirt: A post-office teller took 650 bills from him (in exchange for $650 of unadorned cash). "I am an artist." says Harth, explaining his pursuit. "This is what I do."