I am searching in how to use google maps with ESRI silverlight APIs, i found an article for Mr. Rex Hansen talking about that it is not legal and can be done only using Google Static Maps.
It's really not legal for directly using google maps' tile images in a third party app, including our Silverlight API. However, technically speaking you're possible to do this job easily by extending the TiledMapServiceLayer class, please refer: http://help.arcgis.com/en/webapi/silverlight/apiref/ESRI.ArcGIS.Client~ESRI.ArcGIS.Client.TiledMapSe... If you are not interesting in the details, and just want to see the result in your Silverlight app, technically, you can try Portable Basemap Server: http://www.arcgis.com/home/item.html?id=48bf53da123e442ab8ac9aed52747552 It's provided a REST service url of google maps and this can be directly using in ArcGIS client api, such as ArcGISTiledMapServiceLayer.