I think word filtering is a useless endeavor for anyone that goes to public school.
Also, a URL provided by Tigairius:
http://graal.net/index.php/Creation/Dev/Word_Filter
Though that specific Word Filter doesn't seem to be controllable by the client (which is something I would demand). I personally would prefer a scripted version rather than a built-in version so that the script can be adjusted to work with the playerworld's system.