Three billion iPhone apps have been downloaded from the iTunes app store

"Apple® today announced that more than three billion apps have been downloaded from its revolutionary App Store by iPhone® and iPod touch® users worldwide.
“Three billion applications downloaded in less than 18 months—this is like nothing we’ve ever seen before,” said Steve Jobs, Apple’s CEO. “The revolutionary App Store offers iPhone and iPod touch users an experience unlike anything else available on other mobile devices, and we see no signs of the competition catching up anytime soon.”
iPhone and iPod touch customers in 77 countries worldwide can choose from an incredible range of apps in 20 categories, including games, business, news, sports, health, reference and travel."
Source: Apple press release, 5-1-2010
Note - The first billion downloads took nine months, the second took five, the third took just over three (28th September - 5th January)