When the Port Authority of New York and New Jersey asked developer Larry Silverstein to liven up his construction zone at the site of the former World Trade Center, he responded by inviting 50 street artists to cover more than 20,000 square feet of corrugated metal and walls with graffiti. Faith Salie talks with some of the artists who have made their mark on the site, with every painted surface telling a story of mystery, acceptance and love.