kall, that forum you linked does not work.
When I click on the Facebook link at the top, I get a JS error and it goes to the facebook.php page with no Facebook Connect button.
I have it "working". I disabled the process_templates_complete plugin hook and added the following code to the common footer template at the bottom:
Code:
<script type="text/javascript" src="http://static.ak.connect.facebook.com/js/api_lib/v0.4/FeatureLoader.js.php/en_US"></script>
<script type="text/javascript">FB.init("YOUR_API_KEY_HERE");</script>
The reason I had to working on some of my forums is because I have a fan page badge on the page. So sites without the fan page badge would throw the JS error. The above is work around until we can use another hook to output the above JS.