It would be nice to have ArcGIS Online update the WMS service to be OGC Compliant. See email below
Unfortunately we cannot consume AGOL layers unless they are tiled caches. ESRI hasn't set up their AGOL wms to be OGC compliant, meaning that that while AGOL can consume an OGC wms, a non-ESRI product can't consume an AGOL service.”
It seems like AGO is still not that flexible?