Well let me be utterly honest about one thing, there is no need at all for Silverlight. Silverlight is just trying to repeat Flash and for the web you would be crazy to use Silverlight over Flash.
10 years ago I worked on a Flash project which connected to a back end Apache system using JSP. We had interactive client to server connection using multi-media, no page reloads, and great graphics.
What Silverlight offers is state of the art 1999. For most projects you should consider Flash first.
But Flash is Adobe now, and Microsoft could not use the best client platform in the world if it mean giving that to Adobe. So in short Microsoft rebuilt Flash, and from what I can tell in Expression Blend it made a pretty piss poor version.
BUT, the ability to lay out objects on a Grid now, and apply actions to objects in any .NET language, and the potential to connect this to SharePoint.
I think I might just be in love.