13% of American internet users bought virtual goods in the last 12 months

"Research and consultancy firm, Frank N. Magid Associates, and PlaySpan, the global leader in monetization solutions for online games, virtual worlds, and social networks, today announce their second annual study that evaluates attitudes and behaviors of media and entertainment consumers on virtual goods expenditures.
According to the study, 13% of the overall population surveyed reported that they had bought virtual goods in the last 12 months, with the mean of digital good purchase up 14% from $87 in 2009 to $99 in 2010. The median of digital goods purchase is $50 in 2010, a 67% improvement from $30 in 2009. In terms of the heaviest concentration of digital goods buyers, iPhone owners took over the #1 spot, moving from 28% of iPhone owners in 2009 to 43% in 2010. Virtual worlds came in second place with 41% of regular visitors having bought a digital good. Of weekly handheld and mobile gamers, 33% and 32%, respectively, bought digital goods. Overall, more than one-fifth (21%) of those who bought digital goods said they plan to spend more in the next 12 months. Among ways to purchase digital goods, 16% of digital goods buyers say they have used Facebook Credits."
Source: Press release from Magid Associates and PlaySpan, 27th May 2010
The press release also includes lots of data on the demographics of buyers, and the sorts of places (games etc) where they are buying.