Even if the employee is going to a competitor, it's nice to announce the departure in a gracious fashion. It's also an opportunity to notify employees of any social gathering surrounding the event.
From: Jim Karter
Subject: <subject of memo>
<Name of departing employee> <is leaving/has resigned> Abc Company to <description of what employee will do>. <First name> has served as <job title> for <duration of last position>.
We're sorry to see <First name> leave and wish <him/her> well in future endeavors.
<Optional description of farewell gathering.>