Found at http://dev.jtsage.com/jQM-DateBox2/demos/api/events.html, under "Public Functions"
$('element').datebox('callFormat', <format> <date>)
As applied to your [updated] fiddle:
var dateObject = $('#mydate').datebox('getTheDate'),
theDate = $('#mydate').datebox('callFormat', '%m/%d/%Y', dateObject);
alert(theDate);