If you are talking about doing this from a theme, I actually played with this a little bit. I downloaded the Post Word Count plugin, which by default is designed to show the total word count for all posts. I took the contents of the plugin file itself and pasted it as a function in my themes functions.php file. I then added
<?php mdv_post_word_count(); ?> to my index.php file and it did indeed show my total ( what I assume to be my actual total ) word count. I imaging you could place it in any template in your theme and then style it as you like. The instructions for per post or all posts word count are in the readme file in the plugin. Talk about quick and dirty. I feel a little like I stole it! ??
