jQuery Ajax Callback Failing after WordPress 3.0 Update

Question

I have a bunch of jQuery Ajax calls on my WordPress website. After making the update to WP 3.0, I've noticed they aren't working properly anymore.

The PHP server script is getting called, because the action takes place (Ex: deleting a record in a database, which happens successfully). The issue is that the success callback isn't firing. The error callback is firing, and returning "[object XMLHttpRequest]" when I alert the response.

Is there something that has changed with WordPress 3.0? I use the wp_deregister_script and wp_register_script WordPress functions to load the version of jQuery I desire (1.4.2). So, it should still be loading the same jQuery it always has, and ignore the jQuery that comes with WP.

The site is a membership site, so it is hard to show examples, but you can find a page open to the public here... http://www.learnwake.com/requests. Also, the main javascript file that's adding the Ajax functionality for that page can be found here: http://www.learnwake.com/wp-content/themes/learnwake/scripts/requests.js.

Answer 1

Finally got this resolved!

I was including wp-blog-header.php in my PHP AJAX script. I'm not sure if WP 3.0 changed the way they handle 404's or what, but when I changed the include to wp-load.php, everything worked, and the WordPress environment that I was using in the script, $wpdb object and some constants, were still imported and usable.

A better description, and the original answer can be found here: Original StackOverflow Answer