I looked at the transfered files using firebug and noticed that several files are transferred multiple times:
2 times english XFBML (http://static.ak.connect.facebook.com/connect.php/en_US/js/Api/CanvasUtil/Connect/XFBML)
1 time localized XFBML
1 time english connect.php (http://static.ak.connect.facebook.com/connect.php)
1 time localized connect.php
Is there any way to avoid this?
Also, there seems to be an incompatibility with the "Facebook Like" plugin (neither plugin is working once both are enabled).