Ben - Less than one quarter of the samples in the current Interactive Silverlight SDK operate in the WGS84 spatial reference. While map content in WGS84 tiled services on ArcGIS Online is not being updated, and some are being retired, the functionality demonstrated in the Silverlight samples remains valid. In fact, in some cases working in geographic coordinates has provided a more cognitive example for understanding functionality (e.g. manually creating graphics). That said, we are and have been updating samples periodically to reference basemap layers in Web Mercator. This will continue where appropriate. The Address Matching samples you mention in this post have been updated to use a Web Mercator basemap. Unfortunately the locators hosted by ArcGIS Online are still operating on ArcGIS Server 9.3.1 servers, so the simplest solution is not yet available when using them (merely setting the output spatial reference for the locator task). Hope this helps.