Last year's Rexer Data Mining Survey reported that R is used by more data miners than any other tool. Here's some background on the survey:
The survey is conducted annually by Rexer Analytics. All analytic professionals are invited to participate. Everyone’s responses are completely confidential. All reporting of the survey findings will be done in the aggregate, and no findings will be written in such a way as to identify any of the participants.
A summary report of the survey results will be freely available to everyone. If you want to be sent the summary report, there will be a place at the end of the survey to leave your email address. Questions about this research and requests for last year's 37-page summary report can be directed to DataMinerSurvey@RexerAnalytics.com.
The highlights of the 2010 survey results are available now.