The first phase of this project involved research with scientists at various stages in their careers (and other science communicators) and identified the patterns of motivation and perceived benefits of involvement in public engagement activities.
The results of this research are published in an Oxford University Press book:
Holliman, R. and Jensen, E. (2009). (In)authentic science and (im)partial publics: (re)constructing the science outreach and public engagement agenda, in Holliman, R., Whitelegg, E., Scanlon, E., Smidt, S. and Thomas, J. (eds.) Investigating science communication in the information age: Implications for public engagement and popular media. Oxford University Press, Oxford.
The results are also summarised in the freely available published final report on the project: http://isotope.open.ac.uk/?q=node/565
Thanks to Eric Jensen for highlighting this