As others have discovered, the dict:// URL syntax allows you to search for a word or phrase in the Dictionary program. Now that OS X includes Wikipedia search in the Dictionary program, a search using this method will show you any and all results from all four of Dictionary's sources (Dictionary, Thesaurus, Apple, and Wikipedia).
You can type a search into the URL bar directly, such as dict://Mac OS X Leopard, or you can take advantage of this ability in other ways (e.g. QuickSilver).
[robg adds: This hint updates this older hint, which talked about using the dict:/// (three slashes) construct to look up words in Dictionary. However, in 10.4, you had to have the Dictionary app running first. In 10.5, you can use two slashes, and it doesn't matter if Dictionary is active or not; I tested using Camino in 10.5, to make sure I wasn't getting special treatment from Safari.
I was also able to type the URL in TextEdit, control-click it, and choose Open URL from the contextual menu to open Dictionary. I'm not sure what other apps this may work in.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071026002543385