If you're like me, and dislike the rather narrow font used in Dictionary.app, or if you just want to mess around, here's how:

What you see in Dictionary is actually a web view. Control-click on Dicitonary and choose Show Package Contents from the pop-up menu, then navigate into Contents/Resources. In that folder, edit the body { tag in the DefaultStyle.css file.

[robg adds: I would strongly suggest doing this on a backup of Dictionary, not the original. A few of the fonts I tried led to interesting visual issues when using the Wikipedia or Jargon dictionaries. Also note that since this is a regular web view, you can modify any of the CSS to suit your personal preferences. But really, play around with a copy of Dictionary, not the original (and that way, if Apple updates the app, you won't lose all your changes).]