I've found what appears to be a core-code problem with the TinyMCE plugin packaged in WordPress 3.2.1. Below are the problem details and a work-around that restores the Visual Editor on the three blogs I manage.
This occurs only when editing a post or creating a new one. The second file is also missing its parent folder, /wp-includes/js/tinymce/themes/advanced/langs/.
The workaround is to "restore" these files by copying a similar one into the appropriate folders. Steps are as follows:
1. Copy /wp-includes/js/tinymce/langs/wp-langs-en.js and rename the copy en.js
2. Create this folder: /wp-includes/js/tinymce/themes/advanced/langs/
3. Copy your new en.js to the folder you created.
When you're finished, you should have en.js files in the two locations listed above. With these in place, the WP visual editor will begin functioning normally again. I was able to duplicate this in two WP 3.2.1 installs using few plugins and an entirely customized theme.
Administrators, please let me know if there's a better place to paste this workaround or to give the developers a heads up.