Empirical studies about benefits of Q&A sites for programming
There was a presentation on ICSE 2011, at the New Ideas and Emerging Results track, entitled "How do programmers ask and answer questions on the web?". They only had initial results, but they sounded very interesting and promising. Maybe you could contact the authors if you need more info (they're from the Dept. of Comput. Science, University of Victoria, Victoria, BC, Canada). Here is the full reference:

Treude, C.; Barzilay, O.; Storey, M. How do programmers ask and answer questions on the web?. In Proc. of the 33rd International Conference on Software Engineering (ICSE), 2011.

UPDATE: This paper was just mentioned at the Stack Exchange blog:

Chris Parnin, Christoph Treude, Lars Grammel, Margaret-Anne Storey. Crowd Documentation: Exploring the Coverage and the Dynamics of API Discussions on Stack Overﬂow. Georgia Institute of Technology, Tech. Rep, 2012. [PDF]