jQuery 1.5 has dropped and is making its way to a CDN near you. This release sports a completely re-written Ajax module.
Also new in 1.5 are Deferred Objects, a chainable utility object to manage multiple callback queues for both synchronous an asynchronous functions akin to CommonJS Promises.
// Assign handlers immediately after making the request,
// and remember the jxhr object for this request
var jxhr = $.ajax({ url: "example.php" })
.success(function() { alert("success"); })
.error(function() { alert("error"); })
.complete(function() { alert("complete"); });
// perform other work here ...
// Set another completion function for the request above
jxhr.complete(function(){ alert("second complete"); });
For more release details, be sure and check out John Resig’s blog post. Also catch John on Episode 0.2.8 if you missed it.