Unofficially, yes. London's Daily Mail reports Giant George, a 4-year-old Great Dane owned by David and Christine Nasser of Tucson, Arizona, stands 42.625 inches at the shoulder, .375 inches taller than the current Guinness World Record holder, a Great Dane in San Diego named Titan.
The Nassers say they're awaiting confirmation from Guinness World Records.
Update: Giant George Officially Named World's Tallest Dog - Announced by the Guinness World Records, February 22, 2010.
• Guinness Dubs Great Dane, Titan, World's Tallest Dog
• Meet 'Giant George' the 7-Foot-Tall Blue Great Dane Who Could Be the World's Tallest Dog
• It's Official: Giant George Enters Record Books as World's Top Dog