From the New York Times (registration required):
Joseph Pierre Leclerc was a womanizing bounder who drank too much, beat his children and made a habit of marrying within a month of his last divorce.
After years of research, Michael J. Leclerc knew that much for sure about his unlamented great-grandfather, who died in 1968. What the great-grandson did not know - what had him out after midnight scrolling through just-released microfilm here at the northeast regional office of the National Archives - was which of his great-grandfather\'s countless women was living with him in 1930 when census takers knocked on his front door . . .
Such were the prickly personal questions that brought genealogy buffs out during vampire hours here and across the country for the unveiling of information on individuals and families gathered in the 1930 census. Under federal law, this data, which, most juicily, discloses who was living with whom and in what dwelling, is kept secret for privacy reasons until 72 years have come and gone . . .
More. The National Archives has finding aids for the census available online.
Reg Aubry adds: If you want a non registration-required post for the 1930 census story, here\'s one from the LA Times: