How can I change the 'type' of Drupal script tags from text/javascript to text/plain?
I solved, mostly by implementing the theme_process_html_tag method in the template.  Note the PROCESS not PREprocess.  

In my case, the theme was extending the 'zen' theme, which was actually removing all 'type' attributes from scripts in the header (I don't know exactly why).  But I was able to overwrite this behavior in the theme hook mentioned above.