The trick is slightly altering the ad code in two ways.
First - all the Javascript needs to go on one line. This is because the Forum Intro field is treated like a post, and HTML will be inserted at every new line.
Second - the HTML comments at the beginning and end of the script tags need to be removed.
For example, you'd turn this...
<script>
< !--
var this = that;
var etc = etc;
// -->
</script>
<script src="ads.google.com..."></script>
... into this...
<script>var this = that; var etc = etc; </script>
<script src="ads.google.com..."></script>
Of course, the real thing will look a little different.
One major problem with this, however, is that Forum Intro text only shows up when browsing the list of topics, not when inside the topics themselves. This will really limit the pageviews you get.