O'Reilly Radar tracks technology adoption via its annual "State of the Computer Book Market" report. In the latest report of 2011 book sales (amongst all publishers), books about R show a 127% increase in 2011 over 2010:
As a language specifically for data analysis, R isn't in the same league in raw sales as general-purpose languages like C++ or Java, but it does show the most growth of any major language: R is the bright green box in the bottom row of the treemap above. Amongst print book sales for functional languages, R is both the biggest seller and the fastest growing by a large margin (as shown in the chart at 00:29 in this video about functional languages).
O'Reilly classifies R as a "major programming language", along with Ruby, C and Perl, and attributes the Big Data and data science movements to the growing interest in R. 20,072 books about R were sold in 2011 (compared to 10,775 books about SAS). You can see the numbers for all large and major programming langauges in the tables in the O'Reilly Radar article linked below.
O'Reilly Radar: State of the Computer Book Market, part 4: The Languages (via Redmonk's Donnie Berkholz)