Been having the same problem. After upgrading WP to 3.2.1 the TinyMCE stopped working.
After a couple of hours looking around and trying different things, I found the solution:
you need to add wp_tiny_mce( false ); to the function simple_fields_admin_head:
function simple_fields_admin_head() {
wp_tiny_mce( false ); //this is new
// Add meta box to post
global $post;
…
Here’s an article I found usefull:
http://dannyvankooten.com/450/tinymce-wysiwyg-editor-in-wordpress-plugin/
Hope it helps.
