I've now tried the following:

   var customHeaders = function(r){
    //var h = {'a':1, 'b':2, 'c':3, 'd':4,'e':'HI!'};
    var h = {"X-Method":"Ajax"};
    jQuery.each(h, function(i){
     //$.log('r.setRequestHeader("'+ i +'", "'+ this + '");');
     r.setRequestHeader(i, this);
    });
   }

   // METHOD 1: ajaxSend
   $().ajaxSend(function(a,r,o){ customHeaders(r); });

   // METHOD 2: ajaxSetup, beforeSend
   $.ajaxSetup({ beforeSend:function(r,o){ customHeaders(r); } });

I've verified that the code within function customHeaders does execute during runtime. No errors are reported but the headers are not included in the request.

It seems to work when I use ajaxSetup to set global to true.

However,

1. the documentation says this option should be true by default

2. xml.setRequestHeader("X-Requested-With", "XMLHttpRequest"); executes even if global is false, so the X-Requested-With header should have been sent.

3. I custom headers set via ajaxSend should still have worked.

This is the patch I'm using...

(Note: I use the custom header 'X-Method', but this can be used for any other custom headers)

   // Ajax default settings
   $.ajaxSetup({
    global:true,
    type:'GET',
    headers: {"X-Method":"Ajax"},
    beforeSend:function(r,o){ },
    data:{}
   });
   
   // Add custom headers before every call (HTTP_X_METHOD:Ajax)
   $().ajaxSend(function(a,r,o){
    jQuery.each(
     (o.headers || {}),
     function(i){
      r.setRequestHeader(i, this);
     }
    );
   });

Do you have a sample page which demonstrates this? If you look at the jQuery source code you will see that the X-Requested-With request header is *always* set. How are you confirming its absence?

I confirmed its absence using a simple by calling a simple ASP script via Ajax that prints all headers sent to the server.

Since reporting this behaviour I have downloaded the latest release of jQuery and I can no longer re-create it, which leads me to the assumption that a) it's been fixed, or b) it was an error with my code and I've since fixed it.

Either way, everything is fine now...