Well, read from people who made it :)
http://on10.net/blogs/benwagg/Silverlight-3-Beta-Whatrsquos-New-for-Media/
But it appears that some things are still missing, most important is authentication support when streaming from protected WMS publishing points.
Also, better integration with IIS authentication, using same security context, simplified - having single sign on, when web browser is authenticated against IIS, that Silverlight can use that context when dealing with WMS.
Well, maybe in Silverlight 4 :(
