To download the latest version of the product, go to: http://help.arcgis.com/en/webapi/silverlight
Version 1.2 was released March 2010 (build 177). See the What's New document for more details.
The ArcGIS API for Microsoft Silverlight™/WPF™ enables you to create rich internet and desktop applications that utilize the powerful mapping, geocoding, and geoprocessing capabilities provided by ArcGIS Server and Bing™ services. The API is built on the Microsoft Silverlight/WPF platform which is integrated with Visual Studio 2008 and Expression Blend 3. The Microsoft Silverlight platform includes a lightweight version of the .NET Framework CLR (CoreCLR) and the Silverlight runtime - all hosted via a browser plug-in.