Just tackled this myself yesterday. The way I solved it was to insert an empty div at the bottom of the form (<div id=bottomOfForm></div>) then when closing the dialogue, move it's contents into that div.
In my case, the code was something like this:

        // Setup up dialogue box that contains some form fields:    

        $j(#myDialogue).dialog({

            autoOpen: false,

            height: 300,

            width: 350,

            modal: true,

            buttons: {

                Submit: function() {

                    // Move to main form so fields get included:

                    $j(this).parent().prependTo($j(#bottomOfForm));

                    // Submit the main form:

                    $j('#mainForm').submit();

                },

                Cancel: function() {

                    $j(this).dialog( close );

                }

            }

        });