Never mind, I figured it out. I had the variable that contained the widget handle local in scope. Thanks for the help!
BTW, I used the DOM thingy to remove the IFRAME too. But I had to modify the dhtmlwindow.js source code to assign an ID tot he IFRAME tag. It'd be nice to see that in the permanent source code. 
Code:
t.contentarea.innerHTML='<iframe src="" style="margin:0; padding:0; width:100%; height: 100%" id="_iframe-'+t.id+'" name="_iframe-'+t.id+'"></iframe>'
and then when I closed the IFRAME:
Code:
var ifr = parent.document.getElementById("_iframe-mywinid");
ifr.parentNode.removeChild(ifr);
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