What key groups are under-represented, and why? How has their absence affected the Wikimedia projects?

Just a quick comment -- Netmouse, I think it is also true that editors are primarily clustered in Europe and North America: other regions of the world are less well-represented. Which leads to some parts of the world being better-covered than others.

These links might be useful:

http://en.wikipedia.org/wiki/Wikipedia:WikiProject_Countering_systemic_bias is a wikiproject aimed at countering systemic bias, primarily I believe geographically-based.

http://zerogeography.blogspot.com/2009/11/mapping-geographies-of-wikipedia.html is a blog post by Mark Graham, a research fellow at the Oxford Internet Institute. His analysis suggests that countries like the United States are well-covered in Wikipedia, and countries in Africa are least-well-covered. Germany is very dense with coverage; Chad is least-dense. (His work is based on geo-tagged articles, but the basic premise is surely accurate.)

Here is an article Mark wrote for the Guardian, covering the same substance. http://www.guardian.co.uk/technology/2009/dec/02/wikipedia-known-unknowns-geotagging-knowledge