Thank you for reporting this. You must be using WMS service that is lower than v1.3. We'll try to get it fixed in future releases.What you can do is get the source code for Toolkit.DataSources from http://esrisilverlight.codeplex.com/releases/view/39795In the Wms.cs file, GetUrl method, line# 426
mapURL.AppendFormat("&bbox={0},{1},{2},{3}", extent.XMin, extent.YMin, extent.XMax, extent.YMax);
need to be replaced with: mapURL.AppendFormat(CultureInfo.InvariantCulture,
"&bbox={0},{1},{2},{3}", extent.XMin, extent.YMin, extent.XMax, extent.YMax);