Well I do it because users don't always read announcements -- this way it's right there staring them in the face.
Find in your
forumdisplay template:
Add below:
Code:
<if condition="$foruminfo[forumid] == 'X">
<table cellspacing="0">
<tr>
<td>This is only visible in Forum X.</td>
</tr>
</table><br />
</if>
Replace the 'X' with the ForumID of the Forum you'd like the message displayed in.