I was using fb:editor to build a form and things were going well until I wanted to add a file upload input to the form. To do that you need to set the form's enctype to enctype="multipart/form-data", which isn't a supported fb:editor attribute.
My solution was to copy the rendered fb:editor output (pure HTML) and use that as my form markup. I was then free to add the enctype. The styling wasn't quite right though, so I added a hidden fb:editor above my real form to cue Facebook into embedding the relevant styles.