I've been trying to get Cloudmade tiles to work in Cesium but have run into cross-origin errors (Cross-origin image load denied by Cross-Origin Resource Sharing policy. ) both in my local dev server and in the Cesium sandcastle. Other OSM tile sources (Mapbox, OSM) and TMS tiles have worked fine for me. It seems to be specific to Cloudmade.
This is what I'm trying in the Cesium sandbox