Measurement firm comScore released some new mobile US marketshare data today (Jan - March 2010). It shows 30% of mobile users browsing the Internet on their handsets. If we extrapolate that's at least 75 million people.
ComScore uses a smaller number of mobile subscribers, 234 million, as its base than CTIA's 285 million or the 250 million claimed as subscribers of the four major US mobile carriers. Here is the hierachy of handset OEMs by marketshare according to comScore:
The figures above reflect overall US marketshare not smartphones in particular. The most recent smartphone numbers from comScore for the US market were released in April:
Below are the most recent "content usage" figures and activities:
Update: Apple reportedly has a 5% share of the overall US mobile handset market according to comScore (via AllThingsD):