In an interview, Sid Meier projected that a new version of Civilization would be released in 2009, but the date is uncertain. See http://www.gamespot.com/pc/strategy/civilizationv/similar.html?mode=universe
Unfortunately, the version of Civlization: Revolution released in September 2008 has been called Civilization V (I own it, but it is poor when compared to Civ 4 Beyond the Sword). Nevertheless, the new version of Civilization for PC will probably be called Civ 5.