That's a sweet, helpful code. I've added a forumid conditional to only include specific forums:
Code:
<if condition="in_array($forumid, array(x, y, z))">
<script type="text/javascript">
<!--
var answer = confirm ("You are about to post a new thread in the $foruminfo[title] forum. Make sure it is the correct forum for your topic.")
if (!answer)
window.location="index.php"
// -->
</script>
</if>
Thanks, ChurchMedia.

*Clicked install*