I’d advise against using header.php, because your header is also shown on non-content pages such as 404’s; something which is disallowed in the adsense T’s and C’s.
I did it by writing a small plugin, which you’re welcome to. Just create a my-plugins folder within your bbpress root folder, paste the text below into the file and save it as bbpress-adsense.php, upload it to your server and an advert will appear on every topic.
From there on you have the basics for changing the advert to whatever you prefer.
<?php
/*
Plugin Name: Simple bbPress Adsense
Plugin URI: http://boakes.org/talk/tags/bbpress-adsense
Description: An plugin that includes an adsense advert just above the content on the topic page.
Author: Rich Boakes
Version: 0.1
Author URI: http://boakes.org
License: GPL
*/
class bb_adsense {
function wide_advert() {
?>
<div class="bbpress-adsense" style="text-align:center;">
<!-- Paste your advert below this line -->
<script type="text/javascript"><!--
google_ad_client = "pub-9050338238897291";
google_ad_width = 728;
google_ad_height = 90;
google_ad_format = "728x90_as";
google_ad_type = "text_image";
google_ad_channel = "5984252204";
google_color_border = "FFFFFF";
google_color_bg = "EEEEEE";
google_color_link = "990000";
google_color_text = "333333";
google_color_url = "999999";
//--></script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
<!-- Paste your advert above this line -->
</div>
<?php
} // function wide_advert
} // class bb_adsense
// adds the footer so the javascript is loaded
add_action('under_title', array('bb_adsense','wide_advert'));
?>
