From the New York Observer:
According to two people involved in the story, it started on Friday, March 7, when William Rashbaum, a (NYT) reporter of the old school whose outgoing message refers telephone callers to a pager number, got a tip. The nine-year veteran of the paper’s courts and investigations desks was holding a complaint detailing the arrest of four people associated with a prostitution ring; information in the documents told the story of a john in Room 871 at a hotel somewhere in Washington.
Full story of how the NYTimes brought down the Gov here. (And of course Mostly Media has a tip line: spaceygracey at bellsouth dot net.)