Hi,
That layer (sort of) works for me using this code in Sandcatle:
require(['Cesium'], function(Cesium) {
"use strict";
var widget = new Cesium.CesiumWidget('cesiumContainer', {
imageryProvider : new Cesium.OpenStreetMapImageryProvider({
});
Sandcastle.finishedLoading();
});
However, the Stamen service returns gray tiles for the first four tile levels, for unknown reasons, so you'll have to zoom in before you can see anything. You can see the problem in their client as well:
Zoom in and suddenly you're looking at the U.S. You'll probably need to contact Stamen to ask them why their layer is behaving that way.
Kevin