The jQuery .ajax() docs seem to suggest a request with a dataType of json request to an external domain will result in a JSONP call. (Please note bold)

"json": Evaluates the response as JSON and returns a JavaScript object. Cross-domain "json" requests are converted to "jsonp" unless the request includes jsonp: false in its request options. The JSON data is parsed in a strict manner; any malformed JSON is rejected and a parse error is thrown. As of jQuery 1.9, an empty response is also rejected; the server should return a response of null or {} instead. (See json.org for more information on proper JSON formatting.)

Below are some basic examples of how a JSON/JSONP API may be called. In most modern browsers with support for CORS all the requests will succeed. However, a basic JSON request will not be upgraded to a JSONP request as the docs seem to indicate.

(Note: The jsonp setting for $.ajax defaults to callback)

$.ajax({
   url: 'http://date.jsontest.com/',
   dataType: 'json'
});

• This request fails in IE9 (Reason: "No Transport")
• This request will be successful when ran in a browser that supports CORS (Requested via the XHR transport)

$.ajax({
   url: 'http://date.jsontest.com/?callback=?',
   dataType: 'json'
});

• This request will be successful in IE9
• This request uses the <script> transport even if the browser supports CORS
• Is this the "upgrade" that the docs are referring to?

$.ajax({
   url: 'http://date.jsontest.com/',
   dataType: 'jsonp'
});

• This request will be successful in IE9
• This request uses the <script> transport even if the browser supports CORS