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

ArcGIS API for Microsoft Silverlight

it seems that ESRI is releasing the ArcGIS API for Microsoft Silverlight soon as they are discussing it in the developer summit in March 26

Take a look at Introduction to the ArcGIS API for Microsoft Silverlight here

Take a look at Patterns and Best Practices for Building Applications with the ArcGIS API for Microsoft Silverlight here