I must have been asleep in the last few years: the Dictionary.app in OSX is able to search Wikipedia entries directly. At first I thought it was a Snow Leopard feature but is has been there since 10.5. In 10.6 it gained the ability to open links in articles in Dictionary, too instead of opening them in the browser. As looking up Wikipedia articles is a pretty common task for me, I sometimes spend hours browsing referencing articles, this is a nice shortcut.
Shame om me for not realizing this for earlier.