Are you talking about this Print sample? http://help.arcgis.com/en/webapi/silverlight/3.0/samples/start.htm#ExportWebMap.
This sample use our Legend control http://help.arcgis.com/en/webapi/silverlight/3.0/samples/start.htm#LegendWithTemplates. The legend displays symbols from layer renderer. If you modify this sample to apply a different renderer to the FeatureLayer, this will be used by Legend control. If you want to get to the service-defined renderer, when the layer is initialized, you can check LayerInfo.Renderer.
If, for any reason, you need to get the list of legend infos out of the legend control, you can use the QueryLegendInfos method that exist on all layer supporting the legend.
That being said, for your sample, you should be able to use the legend control and to retemplate it to get the expected result (for example with a WrapPanel, there is a sample here : http://www.arcgis.com/home/item.html?id=c673bf3e983b418495eeb0f45bb44c68)