I'm not sure how you could construct a Binding statement directly, you might need to write a converter for this. Label comes from the UniqueValueRenderer http://help.arcgis.com/en/webapi/silverlight/apiref/ESRI.ArcGIS.Client~ESRI.ArcGIS.Client.UniqueValu.... Renderer from the GraphicsLayer can be of any Renderer type that implements IRenderer. http://help.arcgis.com/en/webapi/silverlight/apiref/ESRI.ArcGIS.Client~ESRI.ArcGIS.Client.GraphicsLa...You can try this code after the layer is Initialized and see that you can retrieve this value.
GraphicsLayer MyGraphicsLayer = sender as GraphicsLayer;
if (MyGraphicsLayer.Renderer is UniqueValueRenderer)
{
UniqueValueRenderer renderer = MyGraphicsLayer.Renderer as UniqueValueRenderer;
string label = renderer.DefaultLabel;
foreach (var info in renderer.Infos)
label = info.Label;
}
You can create a converter that accepts layer and value so that when you can break from the loop when a matching value is found (info.Value == parameterValue).