I think this is a different move and strategy from Microsoft to target more platforms with completely new and immature languages. However it is very promising in terms of technology it provides, cross platform cross browser flash like environment for different programming languages. We had .NET in different platforms like ASP.NET in the web server, Windows Forms and WPF in Windows and now we have .NET in the browser. You don’t have to have .NET framework to be installed because it comes with a small subset of .NET Framework in the plug-in.
The platforms officially supported are Mac and Windows of course. In a very early stage (in 2 weeks after release) Mono has been released their implementation of Silverlight called Moonlight. This was quick because of the open source and BSD style license of Silverlight. So it is becoming a real cross platform in that case, just like flash does with more programming languages and techniques.
Silverlight is very good at advertising as well. A new developer platform space for sharing Silverlight projects is open, PopFly. It is the area to put silverlight applications and see what is possible with Silverlight. Moreover it is also possible to get 4GB free hosting space from Microsoft Silverlight Streaming. However we don’t know how much they will charge later on.
Silverlight might compete with flash in very short time. Flash is de facto standard for most of the browsers.A lot of web applications run flash to provide rich client features, now Silverlight come to the area with the power of .NET framework and your favourite languages.