Right now there is no plans to have jQuery updated to 1.4.0 in WordPress 3.0. It is not impossible, but very unlikely due to fact that it does not cover the WordPress 3.0 Scope of Work.

Use jQuery at your own risk and if you do use it, unload jQuery 1.3 first before registering jQuery 1.4.

wp_deregister_script( 'jquery' );
wp_register_script( 'jquery', '<Location of jQuery 1.4>', false, '' );

I will bring this up at the next dev meeting, but as of right now there is no jQuery 1.4 in WordPress 3.0.x.

• WPRecipes – Prompted me to post this ‘warning’.
• Sean Hayes for the original tip.