It seems like everything is Silverlight these days! Not to be left out, ESRI have announced the public beta of their own SDK for Silverlight. Quote:

"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."

Further information, demonstrations, and getting started articles, can be found here.