I'm not seeing window opacity working with IE8. Here is a code sample:
	Using IE 8.0.7601.17514 64-bit Edition and SmartGWT Power: SC_SNAPSHOT-2011-04-11/PowerEdition Deployment (built 2011-04-11).
							
						
					Code:
	
	public class Ping implements EntryPoint 
{
	final Canvas mainCanvas = new Canvas();
	public void onModuleLoad() 
	{
		mainCanvas.setHeight100();
		mainCanvas.setWidth100();
		mainCanvas.setBackgroundColor("palegreen");
		final Window window = new Window();
		window.setOpacity(50);
		mainCanvas.addChild(window);
		mainCanvas.draw();
	}
}

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