Merlin,

MEP has mentioned one quick way to get to the Dictionary, though the interface is slightly clumsy. A more Mac-like way to get the information (esp. usefully and quick if you have one hand on the mouse) is to hover the mouse cursor over the word you wish to lookup and press Control + Command + D. This key combination brings up a nice little display with the dictionary definition; it lets you switch to the Thesaurus and, unlike with Quicksilver, allows you to launch the full Dictionary app to the highlighted word at the click of a button.