I am presently using the YAHOO.util.Event class to manage some onunload events.

var pupped = YAHOO.util.Cookie.get("pupped");
            if( pupped == null || pupped == 'undefined') {                             
                YAHOO.util.Event.addListener(window, window.onbeforeunload ? 'beforeunload' : 'unload', function(e) {           
                    var y = YAHOO.util.Event.getPageY(e);
                    var surveyCompleted = <%=SurveyCompleted%>;
                    if (surveyCompleted == false && y <= 0) {
                        var x = YAHOO.util.Event.getPageX(e);
                        if (x > 20) {    //x has coordinates in IE. we can omit the back button click.
                            showPopup();
                        }else if (x <= 0){ //x is always "0" in FF. we cannot omit the back button click.
                            showPopup();}
                    }
                });
            } else { removePopupListener(); }

I presently do not see a way to handle this with the DNNClientAPI, but if there is a way to try I would like to know of this.  Any advice?  YUI is simply awesome and the code works perfectly in IE and FF.  The only concern is that if I did miss a way the DNNClientAPI may do the same, we could avoid uploading more js.