Blogger Ryan Briggs (he is a PhD student in international relations, with a specialization in international development, at the School of International Service at American University in Washington, DC) is working on a project to analyze New York Times coverage of nine African countries from 1981 to 2008. He has published some of his results on his website, like this one:

To learn more about the project, see his blog where you can also also get country comparisons or specific stories (like coverage of Rwanda before and after the 1994 genocide).
You can also download the whole set of 14 images that he created here.
Via Texas is Africa