Dictionary and Wikipedia have been having issues of late where the only information you will get from Wikipedia in the Apple Dictionary will be what loads into the Dictionary window the first time. When you try to scroll down for more info there will be no scroll bars. Likewise if you enlarge your window you will get only white space.
This seems to have affected both Leopard and Snow Leopard. It appears that this has to do with some 'New Features' that Wikipedia has implemented that Dictionary can't parse. The only solution I found so far that works is to disable the 'New Features' in Wikipedia.
The solution I found on the Apple Support Discussions page is as follows:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100525215150144