In what capacity? A good chunk of server to client communication is done via JSON because it's lighter weight than XML. Server to server still has to be XML because of advanced features of XML. JSON -> JS Objects conversion is embedded in the JS engines of Firefox, Chrome, and Safari.
JSON is preferred by me because the specifications don't require that it all be parsed at once. I wrote the fastest JSON parser out there because mine does on-the-fly analysis. My engine can load an array of 100 thousand JSON objects and not skip a beat. Try that in XML.