I'm trying to learn the basics of Drupal for Facebook by setting up a sample site that uses Drupal 6.22, the FB 2.1,2 SDK, and the Acquia Marina theme used on your drupalforfacebook site. Just about everything I've tried has worked well except, of course, for that all-important first step of logining in. For some reason the fb:login-button tag just won't render anything but the button's text in either Firefox 5.0 or IE8. In comparing the code side by side in Firebig between your drupalforfacebook home page and my test home page, everything that I think is important seems to match between the two pages until I get down to the fb:login-button tag. On your page I see:
<fb:login-button v="2" onlogin="FB_JS.reload();">
<a class="fb_button fb_button_medium">
<fb:intl>Connect with Facebook</fb:intl>
while on mine I just see:
<fb:login-button onlogin="FB_JS.reload();" v="2"><fb:intl>Connect with Facebook</fb:intl></fb:login-button>
BTW, I've tried putting your code as it appears above directly in the block configuration page, with the result that the button appears, but does nothing when clicked-on.
Another interesting difference is that the div wrapping the login code block on your page is bracketed by
<!-- start block.tpl.php -->
<!-- /end block.tpl.php -->
whereas mine is not. I gather something is not firing to activate the login block properly, but I'm out of ideas as to why. Any help appreciated.