The answer here is really quite simple …if the &callback=something parameter is provided, then:

    //JSONP opening
    if ( callback != null )
        jsonBuf.append ( callback + "(" );
    
    //Beginning of standard reply
    JSON data
    //End of standard reply
    
    //JSONP closure
    if ( callback != null )
        jsonBuf.append ( ");" );

If you’re like most people, and JSON-P is being considered long after all of your REST end-points have already been created, then coding a method that simply wraps the existing REST replies within the “something( normal rest reply );” construct is a very fast way of supporting cross-browser scripting for rest clients.

• It appears there’s a performance penalty for using JSON-P instead of straight JSON replies. See http://jsperf.com/ajax-jsonp-vs-ajax-json for more information.
• Just FYI, Jersey already supports JSON-P with a simple annotation of the REST end-points, and a minor wrapping of the reply as noted above. Further, Jersey recognizes the application/x-javascript mine type to return a JSON-P response without requiring the &callback= query parameter.
• JSON-P is a mature enough technology that JQuery, Dojo, YUI, MooTools, Windows WCF, Sencha Touch, IBM BPM, and most other relevant web client and server products now support it.
• Our chosen DataTables also supports JSON-P with the following modification:

  "fnServerData": function( sUrl, aoData, fnCallback, oSettings )
  {
     oSettings.jqXHR = $.ajax(
     {
        "url"       : sUrl,
        "data"      : aoData,
        "dataType"  : "jsonp",
        "cache"     : false,
        "success"   : fnCallback
     } );
  }