A sample MarkerFormatDefinition (VS 2010 Beta 2)

MyMarkerDefinition.cs

[Export(typeof(EditorFormatDefinition))]
[Name("mymarker")]
internal sealed class MyMarkerDefinition : MarkerFormatDefinition
{
    public MyMarkerDefinition()
    {
        this.ZOrder = 1;
        this.Fill = Brushes.Blue;
        this.Border = new Pen(Brushes.DarkGray, 0.5);
        this.Fill.Freeze();
        this.Border.Freeze();
    }
}