The Yahoo! Web Developer Network provides a one page overview of JSON, giving a quick tutorial on JSON, how to get Yahoo! Web services to emit JSON, offering an output of a JSON object literal as well as using a callback function method. Yahoo! also describes how their web services typically translate their XML structures into JSON.
PPK describes the three main data formats for requesting data from the server, weighing up the advantages and disadvantages. In the ensuing discussion the general feeling is that JSON is the most popular and simplest method.