I have a plugin that restricts the categories users can view/edit in the admin. All works fine in WordPress 3.0.1, but as I started testing 3.1 I noticed that the category filter, which is AJAX, allows you to bypass the restrictions. I'm able to filter this out in 3.0.1 because it's submitted via URL parameters. My trouble is being able to $_GET the 'cat' parameter from the new AJAX request. I have tried testing for $_SERVER['HTTP_X_REQUESTED_WITH'], but it doesn't register.
I've asked this question on the wp-hackers mailing list, but no one can give me actual code that helps with my predicament.