If you are running FBML applications that uses the fb:iframe FBML tag, please read!
Facebook has pushed the following code changed in the week of 26Dec2010:
Fixed bug in <fb:iframe/> that could impact developers that are using <fb:iframe/> incorrectly. <fb:iframe/> will only work with URLs within the application’s domain or sub-domain. (r326549)
So, if you are using this tag to display external pages, then it may be not anymore. I have just checked, it is still working at the moment.
There is a thread in Official Developer Forum discussing this: http://forum.developers.facebook.net/viewtopic.php?id=85238