The API for adding sections to the post/page/link edit screens has changed for WordPress 2.5 -- previously you would use, for instance:
add_action('dbx_page_advanced', 'your_function' );
and in 2.5 you are supposed to use the add_meta_box function.
The problem is that if you want your plugin to work in pre-2.5 and 2.5, you need to detect which version is being used. The suggested logic is:
if( function_exists( 'add_meta_box' )) {
add_meta_box( etc. );
} else {
add_action('dbx_page_advanced', etc. );
}
The problem with this is that when the plugin is first loaded, and also during the 'init' action, the function add_meta_box is not yet defined. It is loaded later, from file wp-admin/includes/template.php
My suggestion is to move the add_meta_box function into a different file, such as wp-includes/plugin.php, which is where the add_action and add_filter functions are defined.
Tested in [6914]; I'll add a patch soon to move the function.
