OpenLayers ArcGIS93Rest layer not showing up

Question

So this seems like it should be super obvious, but I can't for the life of me figure out why I can't get any ESRI layers to show up on my map. I've tried different layers and messing around with the options, but all that happens is that both layer names show up in the layers panel, but only the Google layer actually shows up on the map. Here is the code I'm using:

function addOpenLayers() {
    var mapcenter = new OpenLayers.LonLat(-90.8, 46.45)
    var base = new OpenLayers.Layer.Google("Google Satellite", {type: google.maps.MapTypeId.SATELLITE, numZoomLevels:22});

    var olOptions = {
        controls: [new OpenLayers.Control.PanZoomBar, new OpenLayers.Control.LayerSwitcher, new OpenLayers.Control.Navigation],
    }
    var olMap = new OpenLayers.Map("map", olOptions);

    olMap.addLayer(base);

    olMap.setCenter(mapcenter.transform(
        new OpenLayers.Projection("EPSG:4326"),
        olMap.getProjectionObject()
    ), 10);

    //This is the part that doesn't work:
    var esriRoadsUrl = "http://server.arcgisonline.com/ArcGIS/rest/services/Reference/World_Transportation/MapServer/export";
    var esriRoads = new OpenLayers.Layer.ArcGIS93Rest("Roads", esriRoadsUrl, {layers: "0"});
    esriRoads.isBaseLayer = false;

    olMap.addLayer(esriRoads);
};