//I've tried:
//===========

var fff:ArcGISDynamicMapServiceLayer = new ArcGISDynamicMapServiceLayer();
fff.url="http://mtc-gisserver1/ArcGIS/rest/services/Armory_Atlas_SDE_TESTER/MapServer"
fff.visibleLayers = xc;
fff.alpha = 0.5; // the only way I can see the base layer
fff.imageTransparency = true; // doesn't make a difference if this is included or not
fff.imageFormat = "png32"; // doesn't make a difference if this is included or not 
myMap.addLayer(fff);

<!-- And this: -->

<esri:ArcGISDynamicMapServiceLayer
imageTransparency="true"
url="http://mtc-gisserver1/ArcGIS/rest/services/chris_temp2/MapServer" />