Nielsen is saying now that "21% of American wireless subscribers are using a smartphone as of the fourth quarter 2009 compared to 19% in Q3 2009." That would mean that there are currently almost 60 million smartphones in the US, if the number can be extrapolated to the entire mobile subscriber base.
While I believe that we're very close to 20%, the Nielsen figure is slightly too high when considering the entire market. Nielsen also projects that by Q3 2011 smartphones will become the majority of handsets in the US market.
I think this prediction is also a bit aggressive, at least a year too early. While an increasing number of feature phone owners say their next handset will be a smartphone -- "in the last six months and 45% of respondents to a Nielsen survey indicated that their next device will be a smartphone" -- it won't be until 2012 or early 2013 when we cross the 50% smartphone threshold.