"Many Microsoft web properties have moved over to Silverlight, including microsoft.com, and even more are trying the platform out, like the Microsoft Download Center beta. Nevertheless, the larger majority of Microsoft's websites are using Flash, including many new ones. Why would Microsoft put all this work into developing such a great technology, one that it believes is superior to Flash, and not use it completely and totally? Why are there already more third-party websites out there using Silverlight than there are Microsoft sites using the technology?"

Look here, friends.

Wiki Silverlight explained.

Silverlight homepage.