You set Snow Leopard's default text-substitution controls in the Language & Text system preference, in the new Text pane. This pane also lets you set the defaults for typographic ("curly") quotes, word breaking rules, and spell-checking language for apps that use Apple's common controls. (Note that the Language & Text system preference had been called the International system preference in previous versions of Mac OS X, but -- except for word breaking -- did not contain these controls.)
