This is OCLC's press release about the milestone.
WorldCat reaches 1 billion holdings
At 2:21:34 p.m. EDT on August 11th, 2005, Anne Slane, a cataloger for Worthington (Ohio) Libraries, entered holdings information for the book The Monkees: The Day-By-Day Story of the 60s Pop Sensation, becoming the one billionth holding record in the WorldCat database. OCLC congratulates Anne and all libraries that contribute data to WorldCat.
As expected, the march to the billionth holding was a sprint, with mere fractions of a second separating the entries at the wire.