Integrating Facebook Connect with a website, we had a problem where it would work fine under FF/Chrome etc but IE could not display the FBML correctly.  It was still functioning, but instead of the Connect to Facebook button appearing it was only able to render the text, and any other FB content (such as fb:name) was not displayed.

The best part of the day was wasted until we found this http://axonflux.com/fb-connect-not-working-in-ie-f

Make sure that the page where your FB stuff lives has the following namespace declaration:

html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en" xmlns:fb="http://www.facebook.com/2008/fbml"