Herndon, Va. could be Anytown, USA. Day laborers, many of them illegal immigrants, often clogged a 7-Eleven parking lot, creating unsanitary conditions and disorder. After heated debate, the town last month opened a work center for the jobbers. Order now reigns, but the debate rages on as illegal-immigration opponents seek to shut the center. Herndon's actions reflect a national phenomenon about day laborers: Every day, workers gather in about 500 US parking lots and main streets (and increasingly, work centers), hoping they'll be picked for construction or other jobs by drive-up employers, according to a study released this week by...