Last night I was about to post my (lack of) surprise that the uptake of Silverlight seems to have been quite slow despite the unblockable popup MS has on the front page of their website entreating you to take a look - but decided against it at the last moment.
It's Flash on steroids basically. It's supposed to be able to handle high-definition audio, video, vector graphics, etc, etc, etc.
Elements of it have / are being released and developed in at least a quasi-open source manner such as the eclipse4SL tools for the Eclipse IDE and the Silverlight Control Pack which is being released under the MS-PL (Microsoft Public License). However, if you want to get a REAL bang for your buck from a development point-of-view, you're going to need Visual Studio.
I'm viewing it with suspicion really. Sounds like it could be promising ... but definately takes development out of the hands of anyone but pros and uni students with too much time on their hands. Real development tools for it are costly, and I shudder to think of the security problems it could cause given their track record thus far (look up Silverlight Application Storage).