I saw this problem after installing a new wordpress site using 3.3.1. It worked at first but later it just vanished.
I noticed an error in the background, it was a 404 saying that a en.js could not be found.
To solve this i just created a folder named langs in /wordpress/wp-includes/js/tinymce/themes/advanced/. Then i copied the /wordpress/wp-includes/js/tinymce/langs/wp-langs-en.js to /wordpress/wp-includes/js/tinymce/themes/advanced/langs and renamed the file to en.js.
So now the file exists /wordpress/wp-includes/js/tinymce/themes/advanced/langs/en.js and the editing toolbar is visible again.