Nieman Journalism Lab has some more about what's going on, including details direct from Google: http://www.niemanlab.org/2009/06/google-news-experimenting-with-links-to-wik...
“Currently, we’re showing a small number of users links to Wikipedia topic pages that serve as a reference on current events,” as an experiment.
The post also does a nice job of explaining why Wikipedia (and not something like Wikinews, which is striving to do the same sorts of datelined stories as traditional newspapers) is what Google wants to promote.
-Sage (User:Rageoss)