There seem to be MANY possible causes for this problem -- JSCache, the WPLANG setting in wp-config, compression settings, BSD operating system glitch, etc. etc.
After a day and a half of trying to figure it out, I finally located my particular issue -- IT administrator had placed a .htaccess file in the wp-includes folder, which seemed to prevent WP from accessing TinyMCE. Deleted the file and we're up and running again.
Just thought I'd put this one out there in case anyone else has the same issue.