I was looking for the post and get code today (I'm planning on replicating them for the DELETE and PUT verbs... those should be added) and I noticed a funny problem with the following (GET has the same problem):

	post: function( url, data, callback, type ) {

		if ( jQuery.isFunction( data ) ) {
			callback = data;
			data = {};
		}

		return jQuery.ajax({
			type: "POST",
			url: url,
			data: data,
			success: callback,
			dataType: type
		});
	},

You make a check to see if the second argument is a function, indicating that no data is being sent. That's fine but it ignores the fact that the type might also be passed. As a result, the following would have unintended effects:

$.post("/sample/test", do_stuff_function, "text/html"); 

It also doesn't take into account

$.post("/sample/test", "text/html"); 

Which, while probably silly in most cases, is legitimate within the published docs for the functions.

I suspect the following would be a better solution (depending on how you feel about using arguments.length, you could use other methods to identify things):

if (arguments.length == 2 ) {
    type = data;
    callback = function () {};
    data = {};
} else if (arguments.length == 3) {
    type = callback;
    callback = data;
    data = {};
};