I'm working on a Facebook Connect website and some of Facebook's fbml tags require that they be wrapped in serverfbml tags. Their serverfbml tags render them on Facebook's website and embeds them in an iframe. However, if the page background isn't white, it's very clear that it's embedded inside an iframe. So what I've been trying to do is to see if there's a way to make the background transparent.
It looks like, because the background of the page in the iframe actually has a defined color, that might be kind of hard to do. I'm only starting to look into fbml myself right now, but this thread seems to have some useful information, depending on how you've gone about implementing the Facebook functionality on your site:
Their suggestion appears to be to add styling for the iFrame within your fb:serverfbml tags. But like I said, I know there are a couple of ways to implement this functionality, so I'm not sure whether it helps you. It looks like it might?