I wrote an article a while ago about how to add the subject line to an open ticket alert. While you are not specifically trying to do that, the basics are the same. You need to edit the ost_email_template, which is stored in the database.
(http://www.tmib.net/?q=node/324)
I've been meaning to write up a UI to make this a bit easier. Now if only I can find the time. :)