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:
- From your browser, go to Wikipedia's main page and log in. Use the 'remember me' feature (register if needed).
- In the upper right corner, click on 'New Features.'
- Under the 'Take Me Back' section found in the lower middle of the next page, click the 'click here' indicating you would like to turn off the new features.
- (Optional) Fill out their little survey asking why you want to turn them off and update.
[crarko adds: I tested this, and it works as described.]

