For me, I think the problem occurred during the svn switch to 2.5. Looking into the wp-includes/js/tinymce/ directory showed a couple versions or tiny_mce.js. One was tiny_mce.js.r7627. So I renamed the old tiny_mce.js to tiny_mce.js.old and then renamed tiny_mce.js.r7627 to tiny_mce.js and refreshed. That fixed the problem for me.
