WordPress 3.1 uses jQuery 1.4.4, and jQuery UI 1.8.6. You should check your plugins to see if anything broke. If you were enqueueing jquery-ui-core and using the “widget factory,” or something that depends on the widget factory you will have to update your plugin to enqueue jquery-ui-widget instead. In jQuery UI 1.8.x, the widget factory was separated from jQuery UI core. Please examine wp-includes/script-loader.php to see the entire dependency tree.