[N8]

Just do this:

<if condition="$bbuserinfo['userid'] == 0"> 
<script type="text/javascript"><!--
google_ad_client = "pub-4735661945647510";
google_ad_width = 468;
google_ad_height = 60;
google_ad_format = "468x60_as";
google_ad_channel ="";
//--></script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
</if>

and if you want something to display to everyone but guests:

<if condition="$bbuserinfo['userid'] == 0"> 
<script type="text/javascript"><!--
google_ad_client = "pub-4735661945647510";
google_ad_width = 468;
google_ad_height = 60;
google_ad_format = "468x60_as";
google_ad_channel ="";
//--></script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
<else>
member code here...
</if>