Version 0.5.7 of the Shortcode WordPress plugin adds a new shortcode,
totalwords, that counts the total number of words in published posts. Note that the count is only an approximation: the total is calculated in SQL as the sum of
LENGTH(post_content) - LENGTH(REPLACE(post_content, ' ', '')) + 1, i.e. the sum of the length of all posts (including spaces) minus the length of all posts without any spaces (plus one). Words may not be counted properly if they are separated by more than one space, but the result should be sufficient in accuracy nonetheless.
As always, www.maxpagels.com/shortcodes.txt lists all of the shortcodes offered by the plugin, sorted by version.