Shortly after dawn, the day laborers began gathering beneath a San Diego Freeway overpass in West Los Angeles. A house painter pulled up in a pickup, looking for an assistant. He offered $12 an hour. A worker jumped in. The work is undertaken by men from Mexico and Central America. Most are in this country illegally. The jobs, which last only a day or two and pay cash, are all but invisible to the state and federal governments. No one has to fill out paperwork, follow safety regulations or pay taxes. Yet what happens here is far from marginal. The...