It took 28 days to sell the first million iPads

"Apple® today announced that it sold its one millionth iPad™ on Friday, just 28 days after its introduction on April 3. iPad users have already downloaded over 12 million apps from the App Store and over 1.5 million ebooks from the new iBookstore.
“One million iPads in 28 days—that’s less than half of the 74 days it took to achieve this milestone with iPhone,” said Steve Jobs, Apple’s CEO. “Demand continues to exceed supply and we’re working hard to get this magical product into the hands of even more customers.”
Source: Apple press release, 3rd May 2010