According to Nielsen, Caucasian/White Americans lag behind other groups when it comes to smartphone adoption. The data below are part of Nielsen's recent cross-media study (Q2 2012).
Based on data from many thousands of users, Nielsen reported that 70% of Asian American adults now own smartphones, while 62% of African Americans and 60% of Hispanics also do. By comparison "only" half of Whites own smartphones.
The year will probably end at or very close to 60% smartphone penetration in the US. That would mean something like 150 million smartphone users, most of whom would also be mobile internet users.