<html xmlns="http://www.w3.org/1999/xhtml" xmlns:fb="http://www.facebook.com/2008/fbml">
Internet Explorer 7 is very finicky with its processing of custom tags.
If you're using Facebooker, it actually currently does NOT include this in the html tag of the xd_receiver.html -- so if you try to log into fbconnect using IE7 on an app with the default generated file, it will just fail with a white screen in the login window.
TRICKY. But Forums and Google Search to the rescue once again.
I'm trying to blog these little stumbling blocks back because it'll help Google reinforce the right links just a bit more. Programmer solidarity =)