Looks like a bug, thanks for reporting this.In the meantime, here's a workaround:
var topoBaseLayer = new esri.layers.WMSLayer("http://wms.ess-ws.nrcan.gc.ca/wms/toporama_en");
topoBaseLayer.setVisibleLayers(["limits", "vegetation", "builtup_areas", "designated_areas", "hydrography", "hypsography", "water_saturated_soils", "landforms", "constructions", "water_features", "road_network", "railway", "populated_places", "structures", "power_network", "feature_names"]);
dojo.connect(topoBaseLayer, 'onLoad', function(lyr) {
lyr.setImageFormat("png");
});
map.addLayer(topoBaseLayer);