BuddyPress.org

@latenitebooks, This should point you in the right direction…

Example: http://www.ajaxray.com/Examples/charLimit.html
Tutorial: http://www.ajaxray.com/blog/2007/11/09/interactive-character-limit-for-textarea-using-jquery/

Basically, you would apply the tutorial to the found in the post-form.php and the entry.php.