When facebook introduced yet another API last April, they deprecated FBML canvas pages in favor of the iframe flavor. And developers like me breathed a sigh of relief that we'd never have to deal with straight FBML again (not to be confused with XFBML, still in use on both connect and canvas pages).
But alas, facebook has not (yet) gotten rid of FBML for good. There's still the Profile Tab, which is a nice thing for applications to add to facebook Pages. Unfortunately, to support these your app has to produce FBML.