Aaaahh, found it!!!

Facebook documentation: (http://wiki.developers.facebook.com/index.php/Fb:submit)
Creates a JavaScript submission mechanism for a form, which makes image or text links act as Submit buttons. Markup contained by this tag is surrounded with an <a> tag that includes a submit onclick action.

Basically, anything contained within the <fbsubmit> .... </fb:submit> tags acts as a submit button. All of the tags that Facebook uses can be found at http://wiki.developers.facebook.com/index.php/FBML. By the looks of things they have their own set of HTML markup tags. This is to ensure that everyone that develops applications to work with Facebook can integrate successfully.