Yesterday a few of my Facebook apps suddenly stopped working. I did some digging and found something interesting. Facebook has recently been making updates to how their platform communicates with their applications, both on Facebook and externally, through Facebook Connect.
In doing this, they now require any iframe applications running on Facebook.com to include the callback url (the url where your application is being hosted from) to exist in two places in the application’s settings; in the "Canvas" settings page, and the "Connect" settings page.
Placing it on the connect settings page is where the application now references and looks for the cross domain reciever file, which ensures that the iframe app will render xfbml. Thanks for Matt for finding the fix for this (this dude knows his stuff)
Anyways, we’ve fixed the problem before it became and issue. Let me know if you need to build any Facebook Apps.
Facebook claims they have been running scripts to update any application that has referenced the file in your xd_reciever.htm file…but they lied. So check your app.