ArcGIS API for Microsoft Silverlight™/WPF™ is available as a public BETA

as i heard, it was suppose to be after the developer summit, but its been released as a BETA and version 1.0 BETA will be released in the developer summit Match 26.

what Silverlight API Can Do:

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 Microsoft Virtual Earth™ services. The API is built on the Microsoft Silverlight/WPF platform which is integrated with Visual Studio 2008 and Visual Web Developer Express 2008. 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.

Download the Silverlight API HERE

Getting Started with Silverlight API HERE

Silverlight API Samples HERE

Live Samples HERE

ArcGIS Server Silverlight Resource Page