It has become much easier to assess the academic impact of published papers and authors (Google Citations Page* and T. Reuters). However, the assessment of academic impact on public policy is not that straightforward, although it's a crucial issue for Research Institutes, Think-thanks and Funding Agencies.
A team at LSE leads a project on the subject and they are developing quantitative metrics for measuring the impact of research in the 'public sphere'. It's the Impact of Social Sciences Project. self-recommending
*I encourage you to register at Google Scholar Citations. This is one of the best ways for academics to compute citation metrics and track them over time. You only need a gmail account and Google does the rest for you. Besides, anyone can register!
