4.5 Billion apps have been downloaded from the Apple App store - 1.5 billion in 3 months

"To date, there have been 4.5 billion application downloads from the App Store."
Source: Apple press event, reported by The Examiner, 8th April 2010
Previously - 3 billion downloaded (5th Jan 2010) 2 billion downloaded (28th September), 1 billion downloaded (24th April 2009)