I see that someone has already written a Wikipedia widget for the dashboard on the new OS X 10.4 ("Tiger") release for the Macintosh:
http://www.apple.com/downloads/macosx/dashboard/wikipedia.html
In a related development, someone else has written "WikityWidget," a widget that lets Mac users create their own personal wiki that runs locally on their computer:
http://inkspotting.com/wikity/
Unfortunately, the WikityWidget only supports the CamelCase link style, and I think Wikipedia's syntax is much more powerful and flexible.
It would be great if someone could develop a WikityWidget-like tool that supports the Wikipedia syntax. I've been using a local installation of MediaWiki on my laptop to keep various notes for my personal use, but it would be easier to have the same functionality available on my dashboard. Also, it would help to further popularize the Wikipedia syntax, which I'd like to see become the basis for a standard wiki syntax even outside of MediaWiki.
--Sheldon Rampton