//Simple ellipse (green circle)
Ellipse element = new Ellipse()
{
	Width = 50,
	Height = 50,
	Fill = new SolidColorBrush(Colors.Green)
};

//WriteableBitmap current = new WriteableBitmap(50, 50);
//current.Render(element, new MatrixTransform()); //DOES NOT WORK!!! - renders blank transparent image

//But with the constructor works fine
WriteableBitmap bitmap = new WriteableBitmap(element, new MatrixTransform());

//Ellipse captured as raster image painted on a panel's background
Grid grid = new Grid()
{
	Width = 50,
	Height = 50,
	Background = new ImageBrush() { ImageSource = bitmap }
};

Summary for the Render() method says: “Renders an element within the bitmap.”
Render() method didn’t output required result but using constructor instead of the method did do the job. Input parameters were the same.

Maybe Render() method do something else, maybe it is a problem with Vista 64-bit…

Anyway, WriteableBitmap is very useful in Silverlight 3, luckily it works.