I am trying to determine if a user is a facebook fan. I load the facebook JS library and initialize:

<script type="text/javascript" src="http://static.ak.connect.facebook.com/js/api_lib/v0.4/FeatureLoader.js.php/en_US"></script>

FB_RequireFeatures(["XFBML","Connect","Api"], function() { FB.init("my_api_key","xd_receiver.htm") });

FB.ensureInit(function () {     
        FB.ApiClient.pages_isFan(my_profile_id,"some_UID",callback);
    });

However when I call the API client with FB.ApiClient.pages_isFan, I get a JS error -

FB.ApiClient is undefined

I am also using the FBML fan tag to display the "become a fan" button:

<fb:fan profile_id="my_profile_id" stream="0" connections="10" logobar="1" width="300"></fb:fan>

And would like to be notified when either the "become a fan" button is clicked or a user has successfully become a fan.

The business logic is pretty simple - If they become a fan, track it in my database. Then if they try to become a fan again, check with the library if they are a fan and say "You are already a fan" if they are a fan, show the widget if not.