Ten percent of the US population is (a) a very large absolute number and (b) easily enough that you're likely to see an iPad every day if you look around you.

Ten percent of the US population is (a) a very large absolute number and (b) easily enough that you're likely to see an iPad every day if you look around you. Then if you move into San Francisco, Silicon Valley, Seattle, New York, Los Angeles, and similar population hubs - the places that matter in the US - you can expect to see rather higher than 10% market penetration, especially if you then look primarily at affluent people (like engineers and entrepreneurs) in those areas. Ten percent is not a small number here.