It has been a bit of an open secret for the past week, but ESRI have just announced an ArcGIS API for Microsoft's Silverlight platform. The API will be in ArcGIS 9.3.1 and include a Microsoft Virtual Earth interface. Launch is due for the ESRI Dev Summit (March 23-26).

Quote:

The ArcGIS API for Microsoft Silverlight enables you to integrate ArcGIS Server and Microsoft Virtual Earth services and capabilities in a rich interactive application. With the ArcGIS API for Microsoft Silverlight, you leverage ArcGIS Server resources, such as maps, locators, and geoprocessing models, and Silverlight components, including grids, tree views, and charts, in your applications.

With the ArcGIS API for Microsoft Silverlight, you can

• Display an interactive map of your own data.
• Execute a GIS model on the server and display the results.
• Display your data on an ArcGIS Online basemap.
• Search for features or attributes in your GIS data and display the results.
• Search for addresses and display the results.
• Add toolbars to draw polygons and points, zoom in and out, or pan on your map.

Demo video (incl. Mac & Vista), blog signup, etc can be found at http://www.esri.com/getting_started/developers/silverlight.html