The Pew Internet Project came out with survey findings (n=2,277 US adults) focused on US smartphone ownership and usage. Previous Pew surveys about mobile have been flawed because they failed to distinguish between behaviors of feature phone and smartphone owners. This rectifies that problem.
Pew found that 35% of all US adults own a smartphone. However when you limit the base to mobile phone owners, 83% of all US adults according to the survey, smartphone penetration rises to 42%, four points higher than Nielsen's 38%. Then take a look at those in the 18-44 age bracket. The number rises to an average of 50% penetration.
If you then look at income segmentation, the more affluent the person is the the more likely to own a smartphone. Among those making more than $100K per year smartphone penetration is at least 57%.
Pew also found that 68% of smartphone owners go online daily. Earlier this year Google found (via Ipsos) that 53% of smartphone owners are online several times a day.
Yet here's the statistic that I find most compelling: 25% of smartphone prefer their phones to their PCs as a method of Internet access. (There was no published segmentation of this figure by age or income, however.) In practical terms this is roughly 22 million people. In 2009, comScore found that 40% of iPhone/iPod Touch owners (n=7,300) said they go online more often on their mobile devices than via PC.
These data reinforce that "mobile first" should be a mantra for many advertisers and brands.